I'll be coming to Similan shortly and want to take a dive boat 4-5 days trip and do a lot of diving. I'll need to take Nitrox training. How much does it cost in Thailand? I know the prices will vary a little from one place to another but I just need a bulk part number.

Every DC is free to set its own prices, so your question can not be answered.

You will find that also on liveaboards the course is offered, but nearly always at (much) higher prices.

I kind of have the idea how much the US DCs charge - $100-150 without materials but some include a few free Nitrox dives. even though they're not required. Some charge as much as $250-300. So 5500 bht seems reasonable and I know prices will vary in different areas, not as much as in same area but different DCs.

5500 doesn't include any dives with the course, correct?

I kind of have the idea how much the US DCs charge - $100-150 without materials but some include a few free Nitrox dives. even though they're not required. Some charge as much as $250-300. So 5500 bht seems reasonable and I know prices will vary in different areas, not as much as in same area but different DCs.

5500 doesn't include any dives with the course, correct?

Correct, 2 dives are included for 8,750 Baht.