You can try eBay. A while back someone posted an eBay link that listed a bunch of new/like new Topguns for cheap.
I think if you haven't started building your cabinet you should hold off on buying the gun until last.
Guncon3 for the Playstation works like the Topgun. it's only a matter of time before the PC light gun choices get better.
At least that's what I'm hoping for, and waiting on. If I get my cabinet done this winter I'll be buying two LCD Topguns. But I agree, they do seem a little high, but to avoid any screwing around I think they'll be worth it.