Prestwick losses in 2017-2017 period were £7.8m, so it's costing each person in Scotland £1.47
With 678,000 passengers using the airport during that period, you'd need to charge an extra £11.50 per passenger to cover the shortfall. Revenue per passenger is currently £5.49, which appears to be extremely low, however we know how Ryanair hate to pay their way, so increasing this may be tough.