$500 is light for current collector stuff, but buying just about any milsurp is a good idea. Commercial hunting rifles do not hold their value as well and do not go up in value. Pre-64 Winchesters aside.
Milsurp's do. Supply and demand. Supply is limited and the demand is high. Paid $175Cdn for my 1903A4, sans original scope, but with the mounts, 30 years ago. Same amount for my M1 Rifle. The 1903A4 has increased in value by well over 1,000% in that time. The M1 by roughly 500%. The accessories, bayonets, etc. have increased by roughly 500% too.
There's no reason to think that a Mosin-Nagant, in good condition, won't do the same 20 years from now.