Why is my learning centre asking for 18,000 Baht for a Non-B visa when I've heard it should be around 2,000 Baht?

It requires a significant amount of paperwork and effort to apply for a non-B inside of Thailand and a work permit. It's not all that unusual for a company to have the employee pay for the expense of doing those applications. Normally that should be included in the work contract that, that you are going to be responsible for the expenses incurred as part of the visa and work permit.