The CELTA course cost is largely set by the various CELTA centres around the world. A typical CELTA course fee breaks down like this:
Course Fee (£940) + VAT (£188) + Cambridge Fee (£108) = £1,236
Some CELTA schools will be more expensive, other less so, but you should shop around based on price, reputation and location.
The Cambridge CELTA is not cheap, but it will pay itself back relatively quickly once you enter the TEFL job market. I personally think it’s worth every penny.