Hi Neilson,
If you have more than 1500 hours on aircraft certified for two pilot operations you are exempt approved training. This means you can study how you wish and apply for the exams without going through a school. If you don't have these hours you are going to have to do an approved course, which means studying through a school. The school is allowed to reduce the training time from the full 650 hours, this would normally be done in discussion with you.
The cheapest approved courses are just under £1000, you also need to consider CAA exam fees at £65 a paper for 14 papers, assuming no resits, and accommodation costs during the exam weeks and during any classroom phases of the course.