Costs depend on where you are and who you use. We charge 650 Euros for whole process. You need to first choose a company name (something that translates easily and can be pronounced here in Bulgaria!), then check it is available, then register it, pay certain taxes and fees for set up, place advert in State Gazette about company formation, give someone power of attorney to register your company (as you are not in Bulgaria to do this and even if you are need to speak and write the language), open a deposit account for company, place in 5000 BGN to show you have some working capital, complete rest of paperwork (management agreement for company, specimen of signature, company objectives etc), register actual company, change deposit account to current account so you can access money, get Bulstat card (tax ID) and company stamp.
If you are buying through an agent here, they should arrange all this for you.
Rachel