There are no cheap magazine fed guns unfortunately, but if you are willing to search and/or wait for a good deal you can find one for a much lower price than normal. I personally bought a rap4 marker used for around $300, and it works great for me even if it is a bit older.
A great deal to keep your eye out for though, is when rap4 events end. The company will refurbish all the rentals and sell them for very low prices. These basically brand new guns (used for 10 games tops) are around that $300 range, but they will be the top of the line.
My brother purchased one of these guns, and we have both tried it out. The guns work great, and it came with 2 extra magazines. All in all a great deal. The current big game they have planned is Operation: Endwar, which will be in early October. Keep an eye on this game, maybe even participate to try the gun out, and then shortly after, most rentals will be up on eBay.
Good luck, I hope you like the gun, if you purchase it.
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If you are playing Milsim with mag fed, its whatever gun youd like it to look like, as you give up any advantages normal guns have. The conversion kit for a BT4 is likely the cheapest.