For my Sniper cosplay, I went through Evike Airsoft. I like that they have the "Boneyard", a section where they list guns that are defective/inoperative/returns/ones that were on display in their store/otherwise not in brand-new condition. There's probably something in there that you can get for less than you would pay for the one you listed.

Conversely, a couple of Cons ago, I also bought This one, a cheapo version of the Heckler and Koch G36, which is modeled in the game as the "Classic", IIRC. It's canonical and economical and I didn't feel bad about opening it up and taking out all of the pieces that made it operational so that I could make it past the weapons/prop check desk.