Backs are not very expensive. KEH is a very reputable camera broker n is well trusted by most photographers here. Just Google them, you'll be very pleased with the quality of how they rate the equipment. A BGN tagged back will more than likely look new or gently used.
KEH also stands behind their equipment with a very good return policy.
Edit... I just checked KEH, they have a 120 BGN back for $21. Not a bad deal at all.