Hi Regrib,
It would be a better option if you flew in to Bourgas Airport. There are plenty of flights to Bougas. And price for taxi is around 50-60 leva. Second best option is Varna but it is approx a 1.5-2.0 hours drive to Sunny Beach, depending on road/traffic conditions. It`s 90 km. away to Sunny beach. Where Sofia is a very long way away.
The cheap way from Sofia to Bourgas is by train. Note that trains are too old and slow. The journey takes about 7 hours. The nearest train station to Sunny Beach is in Bourgas, so you would need to get a taxi from there to Sunny Beach. Taxi cost not more than 70 leva. Try the Bulgarian Railway Website:
http://www.bdz.bg/ I would say the best option is the coach from Sofia to Bourgas. After arriving in Sofia, take a taxi to the central bus station and from there to travel by bus to Burgas and Sunny Beach in directly to your choice depending on whether tickets and etc. Best to go to the OK taxi desk inside the airport. Should cost you about 10 lev to train or bus station. The coach journey takes 6 hours. You will find the bus station very well organized. The Sofia bus Website is:
http://www.centralnaavtogara.bg/cbs/home.nsf/vPagesLookup/home~en?OpenDocumentThe other options that I would recommend is to book a private door to door taxi transfer to Sunny beach. The taxi journey takes 5 hours as Sofia is 390 km. away. Recommended for transfer to take a look here:
http://airportpickup.tripod.com/price_list.html My recommendation is to this company because I used them many times and always been happy with their service, but I flew only in to Bourgas airport.
Have a safe journey.
Cheers
John