Danny,
The executive summary is this:
Green spotted puffers are one of two species that are difficult to tell apart, but fortunately almost identical in requirements. While hobbyists use names like "green puffer", "topaz puffer", and "Ceylon puffer" for one or other species, retailers and importers don't seem to, at least not with any consistency.
Both need brackish to marine water conditions (opinions vary, but I'd say an SG of at least 1.010), plenty of oxygen, and good filtration. They are fairly large when fully grown, around 10 to 15 cm. Behaviour is inconsistent; some specimens settle in as tolerably good community fish, while others become inveterate fin-nippers. Many specimens will be good for months, and then turn aggressive almost overnight. The sex of the specimens may have something to do with this, with males presumably being more aggressive.
Feeding is a problem. Not because they are difficult to feed as such, but because they need to be given 'shelly' foods like whole shrimp and snails. This serves two purposes. Firstly, these fish need to abrade their teeth since these grow constantly (like rodents). Secondly, these fish are programmed to 'gorge' themselves, and if given too much good-quality food (like cleaned shrimp or mealworms) the resulting production of nitrogenous waste puts a heck of a strain on the filter. In the wild they feed on what biologists call a low-grade diet, that is, a diet high in fibre of a kind, with lots of shell and bones and only some protein. On the other hand, they feed almost constantly, and so their metabolism is designed to work with a constant supply of food.
(Fish-eating puffers are different, and while they gorge when fed, they are otherwise totally lethargic.)
There's some more about puffers and some links to useful resources in the
Brackish FAQ.
The price you quote sounds fair, assuming the fish are healthy. With puffers, it's always worth spending a bit more to ensure you get healthy stock that's feeding well. An ailing puffer is very difficult to help.
Cheers,
Neale