Quickly from memory... (euros)
Prague 150
Corfu 80 (though this seems to have dropped to about 30 last week)
Paderborn 80
Zurich 70
So perhaps £50-£100 is a more accurate range for the bigger places with Customs, outside France and Spain which are generally a lot cheaper. I can't remember what I paid at Valencia. San Sebastian is now closed to non Schengen traffic anyway.
Here we have Norwich and Newquay at £50 and Cranfield at £60.
I think all these prices are steep for GA but they do not bother me on a one-off flight. If I fly for say 4 hours I burn nearly £300 in fuel, and the hotel won't be cheap either; I have just paid 190 euros/night at Zadar and Brussels can easily be 2x that. I don't stay in fancy places but I don't want to stay in total dumps - too old for the youth hostel type of thing
One would not do it if going somewhere regularly. Then you look for some VFR airfield, and file the big one as the alternate. The problem is that if you want an instrument approach, and Customs, you can be a bit limited. A lot of people stop somewhere out of the UK to get a Schengen stop but if they worked out the total cost of that stop in fuel burn etc they would probably not bother.
For Prague, Vodochody is a bit cheaper and now has Customs (PNR).
There are a lot of stories about Gatwick but last time I checked, about a year ago, it was £80 to land and about £400 for the mandatory Harrods Handling.