It was about £60 pp when we did it in April last year. Good value. Unfortunately we didn't see the whales but July/August is peak time for them at April about the worst. If I were around at that time of the year, I'd leap at the chance to go again.
I've been on this twice, and that price sounds about right, because when we bought tickets, it was a 'Valentine's Day special' 2-for-1', and the tickets worked out to be a little over 1000 Baht per person.
Given it was around Valentine's Day, it wasn't peak whale-watching season, and we didn't see any whales, but because of this, we were told we could try again later in the year for free - I'm not sure if this type of offer still stands..? On this first trip, despite not seeing any whales, we did see a pod of Finless Porpoises (and a dead one), as well as a single Pomarine Jaeger among the more common terns.
I took the free trip later in the year in October, and saw a number of whales, including feeding quite close to the boat, which was a great experience. On the return to Bangkok, we also got lucky when we came across a pod of Irrawaddy Dolphins, too. Birds on this trip were just terns, but there seemed to be a lot of Bridled Terns this time around, especially heading out.
Anyway, well worth a trip out in in Bangkok at the right time of year!
I know that this operator also saw Thailand's first Sabine's Gull a year or so back in the Gulf, as well as a storm-petrel (can't remember the name), and they do offer pelagic trips out of Phuket into the Andaman, though I don't think the demand is too high,yet.