Ryanne, I hope to see several responses from agents in your area. Personally, I have seen interest rates all over the board on a contract for sale...from equal to, or close, to mortgage rates on the general market to double (or more). The down payment, credit of the borrower, condition of the house, motivation of the seller will all come into play. Not to mention, the "exit plan" for refinancing out of the contract for deed (if applicable).
Be very, very cautious about entering into a contract for deed....I am quite sure most industry professionals each have at least one horror story for either a buyer or seller on a contract for deed scenario. Certainly, involve an attorney or a real estate broker. Set it up for success for all concerned, from the onset. The pitfalls are far to numerous to lay out on an advice forum; however, if you want some general guidance you can contact me via my profile. Best to you!