If you’re ok with VFR-only then I’d recommend EBZW or even EBST iso EBLG. Much cheaper !
Thanks. I’m pretty sure I’ll land to Liege EBLG – EDFH and EDFM are alternates, although there are few other airports in Belgium and France which I can use in case something goes wrong at EBLG.
If you want to refuel in the aera maybe go to Giebelstad EDQG, just make sure they are open. They have rather cheap JetA1 (currently 2,25 EUR / liter with all taxes private which is well below other places) low fees and IFR procedures without much traffic.
Hahn is well into the three digits. Avoid.
Keep in mind that Mannheim is heavily restricted for IFR landings, and for MEP landings in general. You can’t legally land there unless you have fulfilled certain conditions. See AIP.
The general Frankfurt area is terrible for IFR flyers. There is not a single pleasant IFR airport within many miles.
Saarbrücken might be OK for your purposes.
Few days ago I emailed EDFH to check prices for refueling but haven’t received any reply. That’s pretty strange for a German airport, particularly because they specifically advertise GA terminal. It seems I’ll remove it 
from my list of potential refueling points. EDFM filed as alternate for my EGNH EBLG tomorrow’s flight.
Thanks everyone for great tips!
If you choose EDFM despite the annoying regulations Peter rightly mentioned, I’ll come over and invite you to an alcohol free beer!
Depending on where you are going in UK Hof-Plauen EDQM can be a good fuel stop. IFR, long hours, nice people, Customs O/R, Jet-A currently 1.56€/l all-in.
Of course don’t know where you’re going, but have you considered SIEGERLAND EDGS ? It’s many years ago now, but I used that frequently as a fuel / food / immigration stop flying between the UK and Central Europe. Great place, very friendly and efficient, the fire guys do the immigration bit (1 hour notice) and customs can be arranged on 24h notice.
