Quite a lot of clubs hire their portable track out to raise money. It is normally complete with engine and staff and runs under the club insurance, A few private individuals do as well but check that they have the right insurance,
Costs are a bit variable and depend on what the event is and how many people will be getting a ride, Often a profit sharing model, or a fixed fee if the event is giving free rides or very low charges so say about £100 or £50 per hour. Some work on a donation box only.
Given that the club members running it will probably in effect each be giving £100 of their time it is not on to pay the club peanuts and get 1000 rides given at 50p a go. Helps if you can provide some crowd management etc workers.

Oh and the site needs to be flat. I mean FLAT like a cricket outfield but ok not the actual wicket. Little engines don't go uphill and you can only pack out so many 4 inch dips every 8t length.

Needed to amswer the same question a couple of years ago. Try here for a start:
http://www.miniaturerailwayworld.co.uk/PortableMiniatureRailways.html

Can not make a recommendation as it did not happen in the end.

Do bear the comments made by Bazyle in mind.