Most hotels will arrange it for you - I think we've been quoted £70 this year return - Sofia to Borovets. Also get some indication on this site of charges, but remember you have to book accommodation with them to take advantage of their packages. Useful for costing though.
Normally the price per km is around 40 stotinki for taxis in Sofia, don't know if you must pay the tour back also, so from Sofia to Borovez will be around 70 km, so it could be may be 60 lev for the driver to bring you Borovez and go back without people. But you can use the bus also. The price from sofia to Samokov is 3 lev and from there the taxi will charge you 7 to 10 lev. They say 10 you say 7 may be 8 and he will bring you for 7 or 8 levs to Borovez. Taxi in Sofia from the airport to ...
Normally the price per km is around 40 stotinki for taxis in Sofia, don't know if you must pay the tour back also, so from Sofia to Borovez will be around 70 km, so it could be may be 60 lev for the driver to bring you Borovez and go back without people. But you can use the bus also. The price from sofia to Samokov is 3 lev and from there the taxi will charge you 7 to 10 lev. They say 10 you say 7 may be 8 and he will bring you for 7 or 8 levs to Borovez. Taxi in Sofia from the airport to the busstation will be may be 2 or 3 levs. so all in all not 15 levs for the trip.
If you take the minibus from Samokov to Borovez (1lev) you safe some more money.
The trip needs one hour from Sofia to Samokov and 10 to 15 minutes the rest.