It depends on the organisation - our local rescue centre charge a membership fee of £25 and a rehoming fee of £75, total is £100. Other centres just look for a donation.
Not mine but a friend asked about one of her horses he's a big 17hh WB & she would have been asked for £150 donation, cant remember what charity, I think he was costing her that in feed over 2 months! She never had the heart to send him though - Sorry that was to send him to the Charity for re - homing!
Our local sanctuary charges £100 for a ridden horse and £50 for a companion plus an annual membership fee of £20.
I think ILPH charge something similar.
£500 sounds like an awful lot.
When we rehomed a companion pony from the ILPH, we were just asked for a donation - so I gave £100. You also need to be a member, and I still am even though the pony was put down a couple of years ago. They were such a good organisation to deal with, so caring and checked my pony regularly, I'd never resent contributing to their cause.
Jen, if you are considering re-homing from the 'local' moorland charity - don't - without GREAT care. PM me for details. Don't want to identify the charity on-line.