Bansko is about 160 km from the city of Sofia. There is a good range of cheap direct flights into Sofia.
The transfer to the airport is one and half hours by car or bus.
Car rental can be arranged easily (www.rentacarbulgaria.com.)
Taxi's are cheap ,charging about £12 per head each way. (Banskoexpress.com)
There is a frequent daily bus from Sofia airport to Bansko.