A well used Marushin UZI, full metal w. Original clip and w/o the box - about $250. A mint condition one in the original box, about $500.
As with all collectibles, Classic guns' value really depends on so many variables, things like condition, completeness, numbers made, market conditions and so forth. But, generally speaking, the costs can be prohibitive to the average airsoft gamer, especially for such guns in mint-condition. As an example - a truly mint JAC B.A.R. can go for about $2000-2500 while my Asahi WA2000 DX can fetch up to $4000 on the international market. Of course, I hear the "but I saw one go for $500 just recently" kind of stuff quite often. While it certainly is true that someone may get extremely lucky - it's pretty damn rare:) One has only to go and perform a internet search on, say, a Asahi MG34 - the results speak for themselves (and most hits quite often end up being one of MY guns:) Are they worth the price? Well, to most, absolutely not! But then again - if one could afford to take a Maserati or (insert your car of choice here) for a spin down I-5 through the Central Valley or do the same thing in a Ford Escort - which automobile would you take for that ride:)
For the most part, one would be misguided in bringing such guns to a field unless you used extreme care and were playing a "static' defensive role only). As a collector (and, given my advanced age:) I assume the role of stewardship/preservationists of the majority of my guns and use them on the target range or merely to show to others at airsoft-related events. As to cost - I dunno - At the end of the proverbial day, when I sit back and/or take a stroll through my collection room - I DO know I am a very happy man indeed to have the guns that I do.......