Countryman B3 omnis are in that range price-wise and are maybe a quality step up. All small cardiods in that price range are pretty limited IMO [edit to add: meaning, I think it is impossible to find a small cardiod that outperforms the mics you already have; that's as good as performance gets with small cardiods], whereas with omnis you can get some really strong recordings. Depends what flavor you like; maybe the Audix would be better for you, or you could try to find an AKG ck9x active setup, but that'll be more like $500 probably once you get the power supply. But really, you have the two best options.
I would just keep what you have and save up for a way bigger upgrade, personally. Or upgrade your omnis to the B3s.