That's incorrect. In EU you can't price-discriminate, based on nationality, regardless of the product or service you're offering. Of course, that applies to EU citizens only. Non-EU citizens do indeed pay higher university fees than locals.
Quick Google search: https://www.medicalstudyguide.com/bulgarian-university-tuiti...
Come to think of it, they probably discriminate on the basis of the program language: If you take the English course you have to pay high tuition fees, while if you take the local language program, you don't.
If you're foreigner, haggling at a local flea market, of course it can happen. But in "official" places, this is simply not possible. And yes, that program discriminates based on language. If you take the version in Bulgarian, you'll pay same fees as locals, though you're usually forced to undergo a paid language course before enrolling.