The research found a basket of everyday items cost £37 at the Black Sea resort while the same goods in Ibiza, the most expensive destination, cost £131.
It also found average prices for UK tourists had risen across Europe, partly because of the weaker pound.
The Post Office said people needed to shop around to find the best deals.
In March researchers looked at the cost of a basket of 10 tourist staples - from lunch to evening meals, drinks to sun cream - in 19 European beach destinations.
After Sunny Beach, the goods were cheapest in the Algarve in Portugal at just over £58, followed by the Costa Del Sol in Spain at £61.
The same goods cost £119 in Sorrento in Italy and £117 in Nice in France.
Andrew Brown, a spokesman for Post Office Travel Money, said Sunny Beach's good value owed much to its food prices.
"Over the course of a week's holiday, lunch and evening meals for two will cost less than around £175 in Sunny Beach but this could mushroom to over £600 in more expensive resorts in France or Italy."
Average prices for Britons across the 19 resorts rose 17% in the year to March - partly because of the weakness of sterling since the Brexit vote, but also the rising popularity of some resorts.
The biggest rises were in Zadar in Croatia, up 44%, the Lisbon coast in Portugal, up 29%, and Crete in Greece, up 29%.
However, prices also fell in several destinations, with Sunny Beach down 10%, Mamaris in Turkey down 4%, and Paphos in Cyprus down 0.5%.
Mr Brown said: "These resorts are trying to attract tourists and restaurants and hoteliers are prepared to drop their prices.
"Also, in places like Turkey and Bulgaria the local currency has actually fallen against sterling which makes them even more affordable."
He said UK holidaymakers should do their "homework", as there were large price disparities between similar resorts within countries.
For example, tourists in Spain could expect to pay 25% more for everyday items in the Costa Blanca - at £76 - than in the Costa del Sol.
And at £89, prices in Majorca were 47% higher than in the Costa del Sol, although they were also almost a third cheaper than in Ibiza.
The cheapest five resorts in March were:
- Sunny Beach, Bulgaria
- Algarve, Portugal
- Costa del Sol, Spain
- Marmaris, Turkey
- Paphos, Cyprus
The most expensive were:
- Ibiza, the Balearic Islands
- Sorrento, Southern Italy
- Nice, South of France
- Zadar, Croatia
- Lisbon coast, Portugal