There are many unseen costs anyway. For example, to get a visa d ( if you are from UK) will cost you with legal(notary) fees, police check fees, trips home (at least 2) for related appointments at Bulgarian embassy, associated hotel costs; then costs to support a resident's permit,setting up a bank account (currency exchange fees on €4000); then getting your A level certificates notarised and recorded, and payment for the entrance exam and registration.... I estimate these costs to be well in excess of £1000. But these costs pale into insignificance compared to the burden the student doing all this without the close guidance and administrative support provided by an agency.
An agency, in my experience is essential: Bulgarian is not a language which is taught fornally in UK schools, and the obstacles are many. Agency staff are bilingual.
Different agencies have different ways of doing things. I think the best ones are those who prompt and support, rather than those who lay it on a plate. The former are cheaper.
Finally, in my view, you will need help obtaining an apartment. If you get this wrong, your experience will be harmed. A good agency will save you every penny you spend on them if you get a good deal on an appartment obtained through them.