we got 16-18 per point depending on which resort points we rented.
not sure how they figure how much onboard credit because it was different for each cruise.
You can call them, or go to their website, and they will be more than happy to explain everything to you, they are very helpful and respond very quickly. I had them change my rooms on both cruises with no charge from them just the difference in cabin price.
Basically take the cash price one
DCL website and divide by 16 and that is how many points you put in the rental que.
They have travel agents that can get you on some other cruise lines and Adventures by Disney by renting your points through them.
hope this helps