Could I earn 40-50K in the rural areas of Thailand?

Generally speaking - no. You will earn probably 70% of what you would earn in the capital but bear in mind that the cost of living will be lower outside Bangkok. That said, you do occasionally see jobs advertised for teaching positions in rural areas at 40K plus. They are around.

Ian McNamara, the founder of ajarn.com says. “I used to get quite a few emails from people wanting to teach on the islands or in picturesque towns up north. Guess what - most of the teachers in Bangkok would too. There are a couple of reasons why we haven’t all rushed to take up these jobs a) there aren’t many of them b) the pay’s pretty shitty”

LMDA, who runs the ajarn discussion forum adds “for the most part, wages tend to be lower ‘upcountry’ than those in Bangkok. Anything over 30k baht is considered pretty good, with full-time salaries of 25k baht the average. It’s certainly enough to live on but whether you’ll be able to save anything is up for debate. Rents are generally lower upcountry, and although it’s pushing it to suggest that everything is cheaper, it can certainly be argued that money does go further than it does in a major city like Bangkok.