In my travels, Koh Tao was the cheapest, if you get a multi-day unlimited package (like 10 consecutive days) and dive whenever the boat goes out, two in the morning (deeper dives), two afternoon, and occasionally a night dive. The diving isn't as good as Koh Lanta, but if what you're looking for is a lot of dives to fine tune your skills, then Koh Tao would be an inexpensive option. I dove enough to average about $15/boat dive, equipment included. The break even is two dives a day at about $25/boat dive, equipment included. Koh Lanta has the additional cost of boat fuel, breakfast and lunch, and marine park fees that make the price higher.