Well, if you want a licensed tour leader, then you should expect him / her to charge you at least 300 PLN per day. This is tour leading, guiding, and foregin language service. I can’t tell you whether prices have risen in the last few years, since I wasn’t running any foregin tours recently.

Apart from that, you are expected to pay the tour guide’s upkeep (food, board and travel).
Please note that such “guides” (pilot wycieczki) do usually work for travel agencies, running much bigger groups of tourists, so the costs of a tour leader are much lower per person then.
The good side is that the tour leader would act as your guide. Theoretically, they are forbidden to do that (they do not have local guide’s licenses), but if there are just the tour leader and you (and a few of your friends maybe), then you could pretend you’re all just a bunch of friends.

[Edited] well, it depends then. Especially on how long is that going to take. I think you should look around for some university student - and then negotiate the price directly with him / her.
A few years ago, back in my uni times, I had ran a 3-day tour (ok, this was as pilot only, and I had the license already) for 200 PLN total.
Whatever you can make the other side accept as a fair price, will be fair price. I’d suggest negotiating a flat price for the whole trip, not pay-per-day, assuming you do know how long is this going to take.
Finding an English-speaking student shouldn’t be an issue. However, as for German, this will be a problem - the old records (most of which were destroyed during the war) were written in gothic characters, and these are damn difficult to read for the untrained. I’m afraid you’ll need a very particular student, probably a history with some specialisation, to be able to read the books (IF they exist) and translate the information into something graspable for a non-historian…