How much data will you need, exactly?
Going from low to high amount, these are the offers you will want to take a look at:
MEDIONmobile: 400MB for €4
JIM Mobile: 500MB for €5
Mobile Vikings: 2GB for €12
(all valid 1 month)
For calls to landlines Bulgaria you could look at Lyca Mobile: €0,13/minute.
Should you need a Belgian sim shipped to Bulgaria, let me know by pm