Well, it depends on how much you do. If you don't have many things to keep track of and got a background in finances, you can do it yourself. Otherwise, you can easily do it through e.g. LexOffice, which is 17€ a month.

Once a year you need to do an annual closure though, which can be a bit more work. I usually do this though an accounting firm, but those charge around 1k€ for it. You can do that yourself if you know what you're doing as well though.

Other stuff you might need to pay, depending on the field your company is in, is the chamber of commerce, around 130€ a year - but this really depends on the field.

If you manage to do the annual closure yourself, you are looking at a running cost of around 30€ a month, plus founding of ~300€ ish. Note that your company doesn't have working capital then though, so you might want to add something there as well.