I would suggest posting the Win 97 trench gun serial numbers and photos in the Firearms Research sub forum (near the bottom of the THR page). Other sources for values include Gunbroker, Firearms Bluebook (printed in regular editions), Military Weapons pricing books (similar), collector selling websites such as Joe Salter, Simpson Ltd., Checkpoint Charlie's, and magazines such as Gun Digest.
These rough estimates are for average to VG condition from what I have observed in auctions, books, etc. from the last 10 years or so. Unfired rifles in pristine condition are worth much more. Those in relic (non fireable) condition much less. Training rifles with machined away chambers etc. are usually less but some collectors want them to complete collections--check websights. This is also true if the collector had provenance (evidence such as letters, government forms, etc.) to establish the history of the rifle. Verified military sniper rifles of any military are their own separate market--consult auction sites and specialized military collector sights such as Simpsons LTD for approximate pricing. Military training rifles in .22 are their own submarket as well.
Any that have been altered by drilling and tapping the receiver (other than by the military), cut stocks, sanded stocks to "fix it", defaced cartouches, alterations such as cutting the buttstock to add a comfy recoil pad, adding checkering to the stock or other such "improvements", cutting barrel length and so on are worth much less.
Take the following as a very rough guide fwiw. Price is usually regulated by condition, demand for the firearm, and rarity.
WWI Springfields $700-1000 in average condition--high number receivers and Mark I receivers bring more as do finger grooved stocks or very early 1903's from the rod bayonet era, WWI 1917 Rifles aka American Enfields depending on condition and originality (no aftermarket modifications, finishes, etc. ) go from $600-1000, British WWI era No. 1, Mk. 3 Enfields about $400 on up depending on the maker, condition, and originality, German WWI era GEW 98's from $400-500 on up, Lebels and Berthiers are hard ones but generally about the same as GEW Mausers, WWI era Ross rifles are pretty rare and haven't kept up with those. Carcanos $300-500 are ball park figures. German WWII era Mausers with all markings have gotten quite expensive usually over $1000. Garands are now at $1000 on up even in pretty battered condition, same for M-1 Carbines, Other Mausers from Sweden, Yugoslavia 24/47's, etc. $400-500 on up which is also true of No. 4, Mk 1 Enfield rifles with Savage Lend Lease marked at a slight premium. Aussie WWII era No. 1 rifles bring a bit more due to relative rarity. Finnish Mosins are about the same starting at $400 and Russian Mosins a bit less unless an odd variant. Steyr m95's about the same as Russian Mosins. Odd firearms such as Dutch Mannlichers, Turks, etc. consult a bluebook. Arisakas range from really cheap to expensive depending on whether the mum is present etc. $250-on up.
Oddities such as WWI trench guns, German odd variants, G43 or 98/40's, etc., you better consult the Bluebook or auction sites and of course any NFA items (machine guns or even ones that are easily convertible to select fire) are a real pickle if not handled correctly--consult a firearms lawyer in that case. Bayonets, field gear, and other militaria, I suggest checking out ebay auction prices and IMA (international military antiques, I believe) website for prices.
