The better grade of liner is 904/904 then 904/316 then 316/316, so there will be price differences on the liner prices.
Some installers recommend insulation, some don't as when the liner isn't insulated the tar etc rattles of the liner easier when cleaning.
You might find you need an adapter from 125mm to 150mm minimum liner.
I would say, if the prices are over inflated, they have loads of work booked in ( seems the trend ) so inflate the prices not worrying if they get the job or not.
Some installers recommend insulation, some don't as when the liner isn't insulated the tar etc rattles of the liner easier when cleaning.
You might find you need an adapter from 125mm to 150mm minimum liner.
I would say, if the prices are over inflated, they have loads of work booked in ( seems the trend ) so inflate the prices not worrying if they get the job or not.