"And, if I need to gig at a small open mike or lounge, I can bring the amp and use it to make my keyboard louder there. Etc."
Your budget is a bit low for what you want to do, and 15 watts is not going to cut it live - period. If you're not singing, just playing BIAB and your keyboard, look for a used guitar amp on CL. An 80 watt ss guitar amp will be plenty loud for an open mic.
If you need a bit more, look at some of the PA packages most retailers are selling.
http://www.musiciansfriend.com/pro-audio/peavey-audio-performer-pack-portable-paRegards,
Bob