Normally its about £50 per person per night, so if its a double room look to get a couple in at £100 per night. If you can only get a single person in, it would be about £60 - 75 a night.
You must bare in mind you aren't registered with the tourist board or had health and safety checks. I would recommend you contact an agency like Liz, as if you serve a dodgy breakfast and you've sold it yourself through a paper, expect to see yourself in court and you would get shot down for running a guest house without being checked.
Chris
BTW I rent a 2 bedroom house in Cheltenham for the whole season which I have for all race meetings of the year. i have it from the Thursday - Monday in October, November and December. Not on New Years Day or 26th Jan. Then I have from the Monday - Saturday at the festival, before having it for Monday - Friday on the April meeting.
I pay £2000 to the owner of the property which he is more than happy for letting his house out for 26 days a year at that.
Its only a 5 minute walk from the course and I rented it off him for the festival last year, I came on the Tuesday, and went on the Saturday. Last year he charged me £700 for the festival week. Bare in mind this is the whole use of a 2 bedroom house over 5 days. If you think someone will pay £135 a night for just a room you will be mistaken. The 2 agencies in Cheltenham have long lists of people wanting accommodation and also advertise in the Racing Post as well. Liz's is the more professional and more well known and she organises Accommodation for alot of trainers, owners and jockeys for the festival.
She would be your best bet if you wanted to guarentee renting out your room for the meeting.