This page compares food prices in Bulgaria with the EU average using official Eurostat Price Level Indices, where EU = 100. Below 100 means cheaper than the EU average; above 100 means dearer.
Food is below the EU average overall, but the picture varies by category: dairy and eggs and oils and fats sit above the EU average, while meat, bread, fruit and vegetables are markedly cheaper.
This is official statistics from the Eurostat–OECD Purchasing Power Parities programme — VAT- and quality-adjusted, independent of any single store. See also the Consumption view.
Source: Eurostat — prc_ppp_ind_1.