I sympathise, sealman - but maybe next year might be better? I've just found flights for 2-weeks random-date travel next June, London LGW-Sofia return for GBP 76, plus of course, a bus fare and some extra travelling time.
Definitely NOT as convenient, admittedly, especially if you are travelling with a young family or have a disability, for example - but if you want to do it on the cheap, then that is the way to go and it certainly won't be as easy.
I'm not in any way suggesting lack of thought on your part as, of course, no one can ever foresee the future actions and pricing policy of airlines with any degree of accuracy.
But that was certainly one of the aspects that I considered and as a result I finally decided to buy property in and near Sofia, with good transport infrastructure between them.
That said, I am usually travelling between Russia and Sofia and thankfully flight prices are now (if I search carefully and book some time in advance) generally good value-for money IMHO, (about GBP 200 rtn). But it hasn't always been that way and still isn't, sometimes....