It’s amazing how little a cab cost in Bali, we hired one for the day for $30.00 USD but that included shopping and the driver gets a % from the shops he takes you to.
There were some street vendors in Kuta and Denpasar selling dive trips to there, if you booked thru them transportation would be included. I don’t know which would be cheaper. You might want to contact some of the Bali diveshops listed on ScubaEarth