I don't know about the rest of the country, but diesel prices have been up and down like a yo-yo over the last six months in my area. The lowest has been around 2.53 leva and highest was 2.85 leva...yesterday it was 2.68. Like most people, I fill up on the cheap and top up whenever I come across a low price.
You can buy cheaper fuel at some of the smaller, independent outlets, but some of them sell absolute rubbish..probably ok in an old tractor, but most modern diesels don't handle it well. If you use places like OMV, Lukoil, Eco, Shell or Petrol, you shouldn't go wrong.