Hi Linda, you might find this link useful
http://www.thedonkeysanctuary.org.uk/foster-a-donkeyThere is loads of info on their site and you can just ring and ask to speak to someone as a 'potential' fosterers. They don't put any pressure on you at all and they offer free training days whether you take on their donkeys or not.
Re costs, having had horses forever, the donkeys seem cheap to me

However, for the uninitiated, costs will include provision of grazing, stabling/field shelter, fencing etc. if not already in place. Then there's vet insurance (the donkey sanctuary have an arrangement with a company that offers it cheaper, so I have that as it was outrageous with the horse ins..company) they need a farrier regularly, every 8-10 weeks but it's much cheaper to do than for my horses. They need worming but the donkeys sanctuary will do FEC counts for free so that we don't have to worm unnecessarily. Their teeth need rasping every 6-12 months, depending on the donkeys. I have mine done annually now as they are fine and I combine this with their annual vaccs so as to only pay for one vet visit. They eat barley straw which is much cheaper than hay. They do need a bit of hay though in the winter but nowhere near as much as the horses. They tend not to need rugs unless they are poorly or elderly.
Grazing does need to be restricted because they can get fat quickly and easily and it can be hard to shift the excess weight. They educate about this though, so once you know, it's easy to manage, you just have to be vigilant and never give them access to too much grass. I was very worried about this at first but it has been easy enough to manage.
Hope this helps.