You want curved or straight? Best deal going on NIB curved ones is through a guy named Gordon Miller. His website is http://www.hkspecialist.net/mags.htm . He sells them for like $49 bucks a piece. More than an AR mag, but I've seen them listed in places for $69 a piece. You can get HK stick mags for under 20 bucks. I think cheaper than dirt is selling them right now. Stick mags are original design and tend to jam with hollowpoints, from what I read. I've never used a MP5 stick mag, only curved. I guess they redesigned them for a reason.
You still have these stick mags lying around? When I come up to pick up a receiver, I'll snag one or two of these to try out.
And, original poster, I forgot about it, but I have two aftermarket MP5 curved mags that I'll let ya have for 15 a piece if you want them. They are okay plinker mags, if that is all you are looking for. They are the same ones you get from blackmarketparts, and the same ones that come with a Coharie Arms carbine. They just sit around and collect dust and take up space.