Can't say for sure what a 5150 is worth, but $200 isn't out of the question for a minty one. Given the recent attention being given to boomboxes lately by pop culture influences like Lyle Owerko's book (and the CBS Sunday Morning feature about it), t-shirts at clothiers like Urban Outfitters, commercials for Progressive Insurance and others, and - of course - this website, we probably shouldn't be surprised that the demand and prices for boomers are growing. What was once a niche-hobby is quickly turning into a fad with mass appeal. After all, eighties teens are now at an age where they can afford to recapture this part of their youth.
For those of us who got into this hobby a few years ago, all of this should come as good news, because our collections are turning into "investments." For those just getting started, the higher prices may get in the way of accumulating a quick, high-end collection. But it should also ensure that good boxes will continue to be available... especially from older collectors ready to "cash in."