Your main problem is the latency the soundblaster card will induce on anything played through it...

This will result in, even if you COULD find a great softpiano at under $100 (I have my doubts), a loss of 'feel' and a sense of disconnectedness with the sound. Latency is a groove killer. If you can't get down to about 2-3ms total latency (which is about hardware levels), you lose the sense that you are actually 'playing', and start to feel like you are unconnected with the sound.