In the $100 range there ain't much with acceptable pianos (that's my humble opinion - but I use either Kurzweil or samples)
I think the Roland JV1010 has better piano than the Proteus (not great, but better than the Proteous) and the drums are better too - I suspect you could find a used one in the $100 range. The JV101 can accept an expansion card (so you could install a piano card - if you can still find one) although the cards would likely cost more than a used 1010.
The plus of the Proteus - it has 6 outs (the JV 1010 only has 2 outs). Depending on how many sounds you plan to trigger and how much seperation you need/want - only 2 outs in limited).