I can't stress this enough. Get a good instructor who is motivated, has a good reputation and who's possibly experienced. I would definitely pay the extra if they are well known for a good pass rate, excellent knowledge base and overall quality in the instruction.

In my experience, the more expensive instructors have in fact been the best ones to go to. They generally have more experience and are more invested into the job instead of just doing it to build time for the airlines or whatever the reason. It could be different elsewhere though.

Having said that, just because you goto an "expensive school" doesn't always equate to better instruction so definitely do your research and ask around if they are any good. Hopefully they've been doing it for awhile too. Usually people at a school will be honest with you if you ask for the better instructors if you don't know anyone there. Experience doesn't always equate to better instruction either so that's why you need to ask around and get a good feel for everyone.

Just do your research, ask around and don't be afraid to switch instructors or schools if you don't mesh with them or you don't think it's working. At the end it's a business, your money and your career you need to look after.