Cost per month through Rental Rewards - $31.30 (assuming Amex is charged the same amount - I couldn’t find info from website) and assuming you pay once a month.
If you have Amex Explorer card, earns 2 MRG points per $, you get 2600 pts per month, which are worth $52 if you value them at 2 cents per point (cpp).
If the value of the points you earn is more than the extra cost you have to pay by going via Rental Rewards, its is worthwhile. However, essentially, you are ‘buying’ points. It comes down to how much you are willing to pay for the point of your choice.