Fitting a hob with a flex is not allowed you can only use rigid or semi rigid copper
using a flexi is not against the gas regs, (some MI state solid pipe must be used some dont) although not best practice, the main criteria for not using one is what temp surface will come in contact with it, ie the back of an oven, built under gas ovens are fitted with a flexi as the back of the oven is designed to accomodate the flexi, so theoretically a hob under a cupboard is perfectly ok to use a flexi as long as it isnt likely to be disturbed by drawers etc. just no need as it is as easy for a RGI to use copper.
cost is too variable £50-£125