As someone who has tinkered with a few milsurps, here is how I see it.
A) Anything in .30-06 bolt actions will be easy to find ammo for it--you will find some South American Mausers using the round or U.S. firearms. These will generally be higher in price for the firearm but cheaper for the ammo. There will be reloading options galore.
B) Anything in 7.62 x54r will have cheap steel cased Russian Ammo and adequate brass supply. Most of these are Mosin Nagants and that is still a bit cheaper than the British Enfield series and much cheaper than U.S. choices. It is also a rimmed case and the firearms are a bit cheaper. Reloading is not difficult but choices are more limited than for the .30-06 for bullets etc.
C) The old .303 British will be more expensive than .30-06 or 7.62x54r (although you could try Wolf's Steel cased .303 now). Like the 7.62x54r, the bullet selection is less than the .30-06 and depending on which Enfield (old SMLE, WWII era No. 4's, or post war No. 4's, bore sizes can vary a bit especially war time production. The P14's are a bit better on that account. The firearms will be between the U.S. military rifles at the high end and the Mosins at a bit below them. That is considering the "average shooter grade rifle".
D) The 8x57, 6.5x55 or 7x57 Mausers. The older small ring 7x57's are usually the cheapest if you go by the Spanish rifles. The Chilean 95's are a bit better made but higher. The 8x57 varies in firearm prices depending on which country the Mauser were made. German mausers generally bring more, then Czech and FN, and so on. Turks used to be the bottom of the bunch but have risen in prices lately. Spanish m43 Mausers are often a good buy as well. Use in WWII spike the price as does WWI era. The 6.5x55 are predominately Mausers (with some Krags being chambered in it), and generally bring quite good prices that often approach U.S. military arms in prices.
Ammunition and reloading for these is pretty decent with many of these rounds still being used and bullets/brass choices pretty good as 6.5 and 7 mm cartridges on the commercial types ensures plentiful supplies. The .323 bullets for the 8x57 is more like the choices for the .303 or 7.62x54 as being limited.
You will find some of these rebarrelled, rechambered, sleeved, etc. into cartridges that may or may not be appropriate. If considering such, become educated about the changes and the quality of these. Some are fine such as the FR-8, some are problematic such as the Chilean sleeved 7.62 NATO model 95 Mausers or the Spanish m93 Mausers converted to 7.62 NATO or CETME depending on your source, and so on. Avoid these if you do not know how to evaluate firearms for safety or at least get a gunsmith's opinion before firing such. The temptation of cheap ammo (usually 7.62 NATO) has led some folks to extoll the virtues of these conversions but the real safety of firing cartridges beyond the original design specifications is something much less than today's commercial rifles that are more accurate, cheaper, and able to handle the pressures involved easily.
E) Everything else: due to military requirements, many countries wanted their own cartridge. In shooter grade firearms, the prices of these often reflect the difficulties in obtaining ammo and/or reloading. Krags, Carcanos in either caliber, Lebel, Swiss, Siamese, Japanese, 7.5 French, 6.5 Mannlicher, and other more obscure cartridges all reflect firearms that loaded ammunition will be more scarce, that will dry up in buying frenzies, and be the last to be produced when the frenzy ends. Bullet selection for reloading can be an issue for some like the Carcanos but not something like the Krags or the 7.5 Swiss/7.5 French. Brass can always be a problem and due to the age of any surplus on a lot of these rounds, I personally would not risk my firearm or health on such. If you must, then pull the bullets and/or reload the brass with modern powders. As a result, you can find a lot of these in quite good shape at a reasonable price but loaded ammunition difficult to find and some components may be difficult to find for reloading.
As far as costs, you can find a lot of these military surplus rifles that have been sporterized, sometimes lightly, for a lot less. Those that have been rechambered, D&T'ed, scoped, restocked into commercial stocks, etc. can often be obtained at prices competitive with the latest whiz bang rifles. What these lack is that you get no guarantees, no recalls for unsafe parts, replacement parts are expensive, dubious quality of sporterizing/and/or safety issues, and it is almost getting prohibitive in cost to restore some of these rifles if key parts like barrels, stocks, receivers, have been altered substantially. If you want the experience of firing these old rifles without caring about full military trim, these are often good buys and are often half the price of the as issued shooter grade types. There will, however, be little upside to price compared with those left in military trim.
Semi-automatics milsurps are a whole nother market where aside from a few WWII era rifles designed from scratch to be semi-automatic such as the Garand or SKS, most were select fire and altered to fit U.S. firearm regulations with some of those alterations resulting in quite horrible firearms. That whole thing is worthy of another post.
I am also leaving out that cast bullets with lower pressure loads are also ideal for a lot of these old warhorses. Bullets can be sized to the bore, cast loads generate lower pressures lessening the risk in firing century old rifles, and accuracy can be stunning even in old worn pitted bores if one spend the time necessary to work up accurate loads for your particular rifle. One issue that is also sidestepped is that many of these rifles used berdan primers which a major difficulty in reloading surplus brass.
My personal foray into milsurps was through a No. 4 Enfield that was lightly sportered. Then a Mosin Nagant 91/30 was bought. I learned to reload for both and both used common components such as bullets and powders. Both had and still do have relatively cheap commercial ammunition loaded for them. From that point, I accumulated but generally focused on cartridge families where commercial ammo was available until recently getting into more obscure stuff. My stuff, by and large, would never be subject to serious collectibility simply because I bought them as shooters or restored them as shooters for historical and mechanical interest. Your interests may vary.
At the present time, I have little interest in casting bullets but there are a few specialty casters that do have storebought lead cast bullets for old military surplus rifles which means that you only need brass and powder.
