Hi Debs,
I have used various insurance companies and only ever had to pay £10 to £20 extra for the RA. It's if you have any other conditions that pushes up the price. My husband now has diabetes type 2 and it was a nightmare getting insured when we went to Canada last year. We used Saga in the end but it still cost £140 extra for his insurance, on top of the basic price. My RA was covered for just an extra £20. It's best to declare it just in case anything goes wrong while you are away, if they then found out you had RA it would cancel out your insurance if it hadn't been declared.
Hope that helps,
Doreen xx