I got my Riva kit today. Truly brilliant.
The carts (blanks) hold a lot of liquid compared to slimline/auto carts, I'm guessing around 50p per fill (from empty) and then less after.
I reckon using fresh carts is way more expensive in the long run. The carts for my old super mini e-cigs were about £1.40 per cart and even as a light vaper, that's at least one a day. I was refilling mine though, like not even 10p because they only hold a few drops. (less than 0.5ml)
Filling a 1.6ml cart is going to be far more costly, but I expect the carts to last far longer, even with the better vapour from the Riva compared to a mini e-cig