Not an expert by any means, but from what I understand from conversations with opticians, when a prescription gets into the extreme end, in order to keep the lenses thin and light the materials involved and the complexity of the lens grinding means that cheap isn't an option. Going to an optician in the far East may be the answer, as their overheads will be cheaper, but you have to balance that against any issue that might arise with the prescription or coating.
My prescription is very simple, -0.25/-1.25, but my lenses cost me £260, put into a pair of RayBan Lennon frames from fleabay, 'cos they're Shamir blanks, with polarising, multipoint vari-focal, DriveWear photoreactive, and multilayer AR coating, so any one of those elements could have gone wrong, and need to be sorted; difficult when the bloke's in Hong Kong or Taiwan, easy when he's sat in front of you and you can bitch about it to his face!
Mine were done by SimplySpecs, in Chippenham. They have an online ordering system, It's how they started, but they have a shop on the estate where I work, which is very handy.