I haven't tried them, but CZ makes some semiauto shotguns in the $300-$400 range, and I've never heard a bad thing about a CZ firearm. Might be worth a serious look!
http://www.cz-usa.com/01.05.php
Can see some street pricing at Impact Guns:
http://www.impactguns.com/store/cz_shotguns.html