Hey Congrats! It is by no means a bad thing to tell BU you're going there and hold onto UCLA's waitlist position. In fact, AAMC guidelines state that NO school can ask you to withdraw from waitlists after May 15 as a criteria for their acceptance. It's part of the way the system works...just wait for UCLA and then reject BU...no matter the date. Now, you might lose your $500, but like someone posted, this is a drop in the bucket in the long run. Congrats again!