Re: 6-24x scope $400 or less
Hard to narrow it down without know what features you want but . . .
Weaver Grand slam or classic series- very good scopes and in your price range.
Hawke sidewinder tactical- has gotten some very good reports and is very popular in the airgun world which means they are tough as nails!
Vortex Viper line but they may be a little expensive and low on the top end.
Bushnell elites- a bit more money though.