You're right, South Thailand is definitely more expensive than North Thailand. In my mind everything is just doubled in price. That's not to say you can't find a 35thb pad-thai or 150thb massage down South, it's just not as easy and common as it is in the North.
I really only know about the South East islands:
- Ko Phangan
- Ko Tao
- Ko Samui
So I'll just stick to them in my examples, but I'm certain that on the West side, Phuket is even more expensive, and Phi Phi similar.
Ko Samui is generally more expensive than Ko Phangan, while Ko Phangan is more expensive than Ko Tao. It all varies based on when you'll be there though.
If you're in Ko Phangan over New Years or a Full Moon Party, prices will generally be higher than Ko Samui for things like accommodation. However, on Ko Phangan if you decide to stay on a lesser known beach or more North (South Ko Phangan is where the Full Moon Party happens), you're likely to save quite a bit of money. It comes at a cost though, with 100thb+ shared taxis needed to get to the South.
In Ko Phangan, specifically the South area of the island, you'll pay about 80-150thb for meals (80thb being Thai food and 150thb western), but that price can go as high as 350-400thb if you go for a nicer meal. The cheapest & best value meal I could find around there was at a very famous place called Mama Schnitzel, where you would get a sizeable Schnitzel (fried crumbed chicken) sandwich for just 60thb. If you stick to that, you can save quite a bit of money as it's always open. Drinks are a little expensive too, with buckets going from 100thb (don't drink those ones if you value your life and health) to 400thb (your own bottle of alcohol), but those prices are given and if you're there for multiple days, make friends with one of the ladies and promise to go back daily - you can get a premium bucket for as low as 200thb if you do so.
Accomodation in Ko Phangan is pretty expensive too, but more than that, very very hard to get (in the South, at least). You're advised to book in advance, and should expect to pay 500thb minimum for a single room. Hostels are obviously preferable, but just as expensive. 300thb would be cheap for one - there are a few that go around that price right near the beach (good location but very small and impossible to sleep). Definitely book in advance any form of accommodation unless you're happy to sleep on the beach / spend 1000thb a night.
The only thing that is similar in price to North Thailand would be massages, but I'd still reserve 200thb for a good one.
Before I finish off this long responsive, I have to recommend you visit Ko Tao, about a 90 minute ferry away from Ko Phangan. It's a diving island. If you're not diving or happy to relax and do nothing, you'll probably get bored, but it's very beautiful, quiet and cheap. Prices are still higher than North Thailand but far cheaper than Ko Phangan. The best value you can get is to book a diving course from Ko Phangan, that'll often include the several-day course, accommodation, ferry to Ko Tao and some other freebies. I personally went with a company called Crystal Dive, which were really great.
Enjoy the rest of your trip and feel free to ask for clarification/elaboration on anything.