If you want to play acoustic guitar in Cambridge, you can probably find a cheap one online for about the cost of your keyboard, or even try to buy one on Craigslist. Mine ran $60, including shipping; it's not a particularly good guitar (either the frets are a little off-tune or the strings are very nonlinear) but then again, it was dirt cheap. I've been to many guitar stores in Cambridge, though, and none sell anything less than $150 that I can remember.