Hmmm. Cheap AND reliable??! You get realistic prices for good work in my experience. Just speak with any old hands you know and I am sure they can tell you stories of cheap work they had done that needed redoing a year or two later. You get what you pay for. Good quality, experienced builders know what they are worth and so do their past customers.
Personal recommendations from people you trust is the best way to find out. I nearly said unbiased but in my personal experience good builders have become my good friends so that would rule out my recommendations! : )
PM me if you can for my 2pennies worth or let us know if you can't PM and I am sure you will get a few suggestions from forum members and see if any of them are for the same people and then get a no obligation quote but remember to always get a few quotes.
Bruixot