wizzair fly from luton, we've paid £64 return (and that's including all the taxes etc). We've got a 3 hour drive to get there though but it's worth it. Car parking for the week is reasonable too.
Went to Bansko last year, our hotel sorted the transfer from Sofia, and the ski and boot hire at a local shop. Probably best to get a price for this from your hotel and then book your flight and confirm the booking if the price is OK. Mal