It would be better either to ask at a chemist or your own surgery for something suitable to take with, if you go to the Doc's on the ship you will have the initial consultancy cost in the region $40/80 then the cost of the injection and this might not be claimable back against your insurance if you could have taken something with before you set off for the cruise ship.
Blyth obviously has no experience of serious sea sickness, when tablets and wrist bands do not work and only an injection does the trick! Can't help with price on Ventura, I'm afraid. but it was £16 in total on Thomson Celebration a few weeks ago.
I am prone to sea sickness as well and took sea sickness tablets with me expecting to need them but when on Ventura in December the ship was so smooth I did not need them. I was surprised. Probably if you were going round the Bay of Biscay you would probably need them. dont know how much an injection on Ventura would cost though sorry.