The two month ranges barely overlap. An unlimited month runs 3,999 to 9,800 baht across the 3 of 4 camps in Hua Hin that price one and 9,000 to 16,500 across the 3 of 4 in Pattaya that publish one, which leaves the dearest month in Hua Hin only 800 baht above the cheapest month in Pattaya. A single group session is 400 to 450 in Hua Hin and 400 to 800 in Pattaya, the same floor and a different ceiling, and the ceiling belongs to the one mega camp. The private hour is the single place Pattaya goes lower, 800 baht at one camp there against a flat 1,000 at both Hua Hin camps that publish a rate.
The gap is not a discount so much as a different product. Three of the 4 Pattaya camps say they have accommodation on site and one of them prints a nightly rate of 1,000 baht, which is the only bed price either town publishes. Hua Hin now has 2 of 4 saying the same, and neither of them puts a price on a bed: one sells nothing but accommodation-inclusive camps and will not split the figure, the other says only that rooms exist. So the three Hua Hin months you can actually read are training and nothing else, and the room stays a separate problem you solve in town. Scale runs the same way. All 4 Hua Hin camps are boutique and only 1 of them teaches anything besides Muay Thai, while Pattaya has a mega camp, two mid sized gyms and a boutique, and 2 of the camps here add boxing and mixed martial arts on top of the Muay Thai.
Season is the thing that actually separates these two, which is not true of most pairs on this site. Hua Hin runs on the Upper Gulf calendar, driest December to March and wettest roughly September to November, and it sits in enough of a rain shadow to stay usable in months that are washed out on both coasts proper. Pattaya runs on the Central and eastern calendar, which is the Bangkok one, driest November to February and wettest roughly May to October with the heaviest of it around September. From May to August that is the whole difference between them: Hua Hin is between its seasons and Pattaya is in the middle of its wet one. In September they are both wet.
The thing people miss
Both towns are thin in different ways, and both are thin on the one point that decides this. Two of the 4 Hua Hin camps do not say whether they have rooms at all and neither of the two that do puts a price on one, and in Pattaya one camp of 4 publishes no price of any kind, so the Pattaya range is really three camps wide. Between the eight camps in both towns there is exactly one published nightly rate, which means neither place can be costed end to end from what the camps themselves put in writing. The 4 camps listed in Hua Hin answer 26 of the 48 questions we put to them on their own sites and the 4 in Pattaya answer 28 of 48.