If you had the ability of purchasing for $300 within the first 5 years of car ownership a Prius Battery Replacement Contract with includes the 12V and the Hybrid Main battery would you? If the battery stops functioning you would be guaranteed replacement. The contract is not transferable if you sell the car. It has no age or mileage limit. This does not exist just seeing if there is a market or interest in it.
In the UK the answer would have to be no. A new 4 year guaranty aftermarket 12 volt battery is £40 -$65 and the HV battery has an 8 year guaranty, and both are transferable.