I've not used it myself, but IKM's SVX gets very good reviews and seems to be the top choice. But yeh, it's a hundred bucks.
You might try the Studio Devil VBA, which has two versions, one for $99 (2 channels) and the other for $69 (one channel).
For free there's the
TSE BOD, which I have used and, well, it's worth every penny.
If you have Guitar Rig or GR LE, there are bass amps in there, and if you've already got it you might as well give that a try.