IMO the most "collectable" and affordable rifles out there the Swiss K-31, and the Swedish Mausers M96 and M38. There are still a ton of K-31's out there for less than $350, they're extremely accurate and about 500,000 made. Simpson Ltd.has pages and pages of them on their site. Same with the Swede Mausers. The longer barreled 96's has a little over 700,000 made and shorter barrel M38 only had about 80,000 made. Both are also extremely accurate can be found in the $400-500 neighborhood, and the 6.5x55 round is a lot of fun.
It's going to really be a matter of what you have interest in and want. From a collectable stand point I don't rate Mosin's high on my list of collectables because there are millions and millions of them floating around. But for a personal collection, or WW2 collection, etc...no collection is complete without one. But for my money milsurps with the best combination of accuracy, high quality manufacture, lower production numbers and affordability are K31's and Swedish Mausers and post WW2 FN Mausers.
"And remember, Abraham Lincoln didn't die in vain, he died in Washington D.C." - Firesign Theatre