Flights with Aer Lingus are approx €140 inc tax only up till mid May,after that Dublin-Bourgas are working out at €200 plus,so if you didn't mind going early May,it might work out more economical,but then you have to factor in times between connecting flights,the hassle,weather in SB at that time and i believe the season isn't properly up and running at this stage, in that a lot of places do not open until the end of May,good luck with your flight hunting