If you use the Search button at the top of the page you should find lots of previous threads that might help you.
If you are a member of one of the two big clubs then it's worth trying them first. Price will vary depending on value, cover required, security measures and location.
I use Cover4Caravans. Can't help on price because I transferred it over from my tourer when I changed it and it does depend on the value of your unit and whether you need to include contents.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.