@Piotr Laskowski7
Welcome to the °µÍø½ûÇø.com forum, congratulations on your new apartment in Bulgaria!
I'm not sure what "offer" would have included a reference to a maintenance fee, I don't think that it's mentioned in your Notary Act, which is the most important legal document of your purchase. If you mean the property listing, agents get this kind of detail wrong all the time (or the maintenance fee goes up).
Many touristic mountain (and beach) buildings can be quite expensive, perhaps even 10-15 euros per m2. My Bansko studio is quite low, about 4 euros/m2. But many Bulgarian buildings might be charging 10-20 lv per month just for cleaning (maybe closer to 2 euros/m2).
There are laws about maintenance, and you are required to pay. And it's always recommended to keep your building clean and in good condition, don't you think? :-) However, enforcement can be slow, so you might get away with it for a while.
Lots of countries like (or require) reserve funds (accumulating accounts against future repairs). This seems very rare in Bulgaria, so here money is requested as a big job (repainting outside, re-doing the roof, etc.) becomes due. Generally, if not enough owners cough up, the complex doesn't have the funds, and hence can't do the repair. Therefore the headline amount (e.g. my 4 euros/m2) is usually just the baseline amount to cover a bit of communal cleaning and lighting, basic repairs, keeping the elevator running, etc.
At our Bansko complex, the owners have wanted to switch to individual water and electricity meters. This is quite expensive, so for each project (electricity meters first, water meters more recently) the cost was circulated, and divided between the owners. This one's a bit of a no-brainer as business rates are higher than individual rates, and an exact usage is better than an estimate/share of communal usage. But, even so, many owners wouldn't (or couldn't) pay up. (Only a few, so there was sufficient funding for the project to go ahead, but they didn't get a meter.)
Also, there is property tax here, so you should go to your local property tax office, register your acquisition, and figure out how to pay (they can give you online access).