Used digi-modded UA5s run around $250. You'll need an optical cable to run UA5 > JB3, ~$15. If you keep your current mics, you'll then need to either get an adapter to run from 1/8" > 2 x XLR, or phantom-mod the mics (
details here). I have no idea what that'll cost, but not a lot.
But, IMO, you're better off upgrading your mics first.