The house you're describing wouldn't be interesting to me for a quilting retreat, unless there are features that you haven't mentioned. Quilting in a garage sounds dreary, and having to trek to it from the main house in bad weather (hot, cold, or wet) could be a problem for many. Also the number of people it can accommodate is quite low. Most retreat sites that I'm familiar with have more to offer. I would suggest that you work with the numbers, determine how much income the site needs to produce to break even (including taxes, upgrades, maintenance, insurance for this type of use, etc.), and then show how many bookings per year at various price points would be needed to meet this goal. If you can get some input from local guilds that is good, but I think it really is up to the buyers to decide whether they can entice people to come there.