You could always try the usual 'cheapest flights' websites, e.g. Cheapflights, Skyscanner, Travel Supermarket, etc.

Cheers will have a go...

Don't expect flights to Bulgaria to be anywhere near the cost of flights to Spain, due to lack of competition/limited number of carriers, etc.

From memory, only BA, Bulgaria Air and the 'no frills' carrier Wizz Air operate regular scheduled direct flights from the UK, from a limited number of airports, so most flights available from regional airports will be tour operator charters.

This year we are going from Manchester to Sofia with easyjet. Price of 3 adult flights £433,return for 3 weeks. Flights to Bourgas at the same time with Balkan Holidays was £1317 in total.

Hi moondust,

In case you aren't familiar with the geography of Bulgaria and are planning to visit the coastal resorts (which was my original assumption), it's probably worth pointing out that Sofia is a reasonably long journey from them - so would mean using cross-country flights, car hire or public transport to travel to the resorts:

From Sofia Airport to Sunny Beach is 433.37 km (just over 269.28 miles)

From Sofia Airport to Golden Sands is 460.8 km (just over 286.327 miles)

Thanks Baldur. Yes we are hiring a car, which we usually do anyway, and intend to take 3/4 days to reach the coast. We are staying for 3weeks and will then drive back to Sofia on the final day.
What we are saving on flight price will more than cover the cost of car hire and accommodation en route.