As someone who's been responsible for maintaining a fleet of rental bikes, I'd also say about £30 for a wheel for a day doesn't sound unreasonable.
As others have said, it probably meant incapacitating one of the rental bikes and then setting up the gears and / or disc to make it work. It took a bike out of circulation and it took a mechanic's time.
It sounds harsh, but the hire places have to make money, because they have to keep their fleet running and they have to pay their staff as well as buying tools, spares, oil, brake fluid etc. before you think about the cost of the rent, electricity, heating, la la la… £30 to get your mate riding for an afternoon is pretty defensible.
: P