"Cost" is the past tense of "cost." If the verb in the sentence that the native speaker corrected was supposed to be in the past tense, then it should, indeed, be "cost." However, if you are speaking in the present tense, then it must be "It COSTS €50" WITH the "s". For example: I had to get a filling at the dentist last week and it cost me €50. But: It costs €50 to get a filling at the dentist. Fyi, I just made up the price. I have no clue how much a filling costs. :)

"The filling costs €5.00". This is present tense. "The filling cost €5.00". This is past tense. Example: Yesterday the filling cost only €4.00, but today the filling costs €5.00.

costs = present tense cost = past tense

Those represent the difference between present and past tense.

Cost is correct. Because "cost" is an irregular verb.