You can fly directly to Salamanca Airport (called “Matacán Airport”). There are daily flights from Barcelona, Malaga and Palma de Mallorca (with Lagun Air and Air Nostrum). Another option is to fly to Valladolid Airport or Barajas International Airport in Madrid, the closest airports to Salamanca.
Regular bus services are available from Matacán to Salamanca and they has an approximately cost of 3€.
More information about bus services: http://www.estacionautobusessalamanca.es/
The taxi service from Matacán Airport to Salamanca has an approximately cost of 25-30€.