We did the same thing last year, only the couple who arrived before us had hired a car so they picked us up from the airport.
Don't know how much the taxi's are but I think they're quite reasonable. There's a link on this website in the menu on the left "book a trip/transport" which might be useful
Google "airport transfers sofia" and you come up with various sites, holiday taxis seem pretty reasonable we have got a minibus pick up for seven people return for £180 for Bansko.
I got back from bansko today and used a taxi to get to the airport. It cost 70 euros
Cheers
Smag
Dont ask why we had to pay in euros. It was the only thing we paid for in euros in the intire trip. I pressume we could have paid in levs, but we took some euros with us from the start and needed to use them up.