£10 or £20 is quite cheap for share. for 3 rides a week i would say £100. goodluck 

 As I have been at university, my pony has had a sharer for the past 2 years to a teenage girl. She had him half the week and contributed £100 pcm towards his costs. Sadly she has now outgrown him and is sharing a bigger horse but we have just found an 11 year old to share him over the next 12 months. I'm hoping it will work out as well as the first share agreement! I always insist on a share agreement in writing (I used the template from the BHS website and then tailored it to suit my situation) as you never know when these things can go wrong. 

As my pony is older (he's 24), I'm less worried about him being 'ruined' - as he is a good doer, I'd much rather he was exercised whilst I am away and I didn't want a full loan as I feel he is too old to move back and forth yards. Also, I find that working out how much your horse costs to keep annually, dividing that by 12, then cutting it approx in half if the sharer has your horse half the week works really well as a contribution rate. I don't include some costs within this like insurance, though so I pay more than half overall.  Whilst horses cost more in the winter, someone sharing him/her might feel they are getting a rough deal if they are not getting much riding but are paying more over those months. Easier to stick to one cost per month all year round that takes into account the rise and fall of expense depending on the season. 

In terms of finding someone suitable, someone recommended to you or someone already at your yard may be the best bet. Failing this, offer viewings in the same way as if you were selling your horse. That way, you will be able to decide whether you are happy with their level of ability whilst they judge whether they like your horse. 

Hope this helps. 

 I think sharing is a fab idea, I've had a young lass sharing my boy for the last 5 months and would really be lost without her now, i often wonder  how i managed before she came along, with working and a young family time can be tight at times and plus my boy kept on some land i rent so everything was down to me..She pays me £10 a week to help towards costs of feed and half of his farrier billlmucks out and poo picks him on her days and she can ride him 3 days a week and i take them out to local shows etc,etc and he gets the work i sometimes just to have time for although i will have to say it did take me a while to come round to having someone else riding him.. She gets to learn things,under my guidance that she wouldn't generally learn at a local riding school( atm dealing with an abcess, politicing, bandaging etc.)  So if worked correctly and with some you trust it can be great.