If you just need a DI and that's it, then for dirt cheap it's tough to go wrong with a Whirlwind.
If you need something to amplify the signal as well, then I'd say grab an ART Tube MP. It really shines as a DI for bass guitar. I can't say enough good things about it.
If you want something that models the sound of playing through an amplifier, then the Sansamp Bass Driver is pretty stellar. Very versatile in the kinds of sounds you can get with it. Can't say enough good things about that one, either.