Ah the old to build a Mauser or not build a Mauser quandry, I go through this a couple of times a year.
I think there are two categories that this falls into, either a low end rifle, or real high end stuff and it makes a big difference in your decision.
On the lower end stuff, remember you can buy a complete out the door CZ or Remington 700 for $450. That isn't very much to work with when building a custom ( or customised ) rifle. It costs less to buy the commercial gun, and the resell value is better in the long run.
The high end stuff is another matter all together. Look at prices on a Griffin and Howe, a Holland and Holland, or an original commercial Mauser or Rigby. Or compare to a branded smith like Jerry Fisher or D' Arcy Echols. These guns are out of most of our budgets. As a based starting point look at a Dakota 76 rifle, I think these are right around $5000. If you are building a Mauser action up to be this type of riifle while I don't think it is a lot cheaper in the long run it will save some money, but only $1000-1500. When you get playing with rifles that cost this much little details make big differences, on what you end up with. Typical upgrades are new bottom metal, a lot of work on the action to get it into a square bridge configuration, trigger, bolt handle, cross bolts, bolt shroud and safety change, plus barrel bands and quarter ribs. And starting with the right Mauser action to begin with is ultra important, no Yugo action is going to cut it. A original 1909, a VZ-24, G33/40 are good actions, or better yet a original banner commercial Mauser action ( big bucks). Then the stock comes into play, and this is a serious chunk of change. I am going through the exercise right now it looks like:
1. Action........$150-250
2. Action work.....$400 min including bolt handle
3. Bottom metal.....$250
4. Trigger ......$80
5. Barrel.......$300
6. Install barrel chambered etc.......$250
7. Bolt shroud safety........$150
8. Stock........$1000 min to $3000
9. Grip caps.......$30 min plus engraving
10. Barrel band swing swivels......$80
11. Front sights.......$100
12. Engraving... whatever floats your boat but it starts at $100
13. Rear sights.....$40-250 depending on your choices
14. Blueing and surface prep..
15. Heat traeting.....$35-75
Now you can whittle on the above list and trim some dollars, and I have made some good educated guesses on the above, as I have done my homework and don't have time to price it all out right now, but my numbers are close only a few dollars off a worst, and some of these a even a little low. But when the above gets done right you will have a very fine rifle, and probably worth at least $2000 when your done, more if you are real picky on your smith and the action you started with. Oh I forgot to add the around two years it will take to get the above done. I have been looking at gunstocks seriously lately $1000 is entry into this and a year seems to be the typical waiting list. Oh I didn't add the price of the stock blank into this, low end is $150 and the high end goes way over a grand, but $300-$500 gets a decent blank to build the stock from. I would schedule 2 years to get the above done right, anything else will have you diisappointed in what you get back or PO'd cause your expecting less time and two years is what it will take.
And no you don't need to get everything on my list done, but remember when you start short cutting the list you get closer to the first scenario, when your done and it becomes of matter of is it worth the effort and dollars. I also think there are better options than starting with an old Mauser action, look at a new Win 70 or a Remington 700 as your starting gun, they will save you some dollars in the long run on machining and custom parts.
Or you can just go out and buy a new Sako 75 or a upgraded Model 70, or something similar and its a quality rifle from the start that doesn't take forever, and $1000 out the door makes it a done deal.
No the day of cheap mauser conversions is gone forever. If you want something special, and are willing to spend both the time and money it is still a option, but cheap and mauser conversions don't really exist amymore.