You need to understand what different types of underlay do for your installation, and what you have been offerered at £10m2
Not all underlays do the same thing in different installs, the type of carpet is important, the deeper and softer the underlay, the more pressure, traffic marks etc (but it will feel soft) the firmer the underlay the better the wear and appearance retention.
If it's a rubber waffle or PU Foam style underlay (i.e. cloud 9) walk away, your being ripped off.
You should be able to source a fantastic natural felt underlay such as Axminster Axfelt65 through a good independent shop for less than £8m2 with room for negotiation easily.
If you have 'blindly' been offered underlay at £10m2 and not been shown a selection of different styles of underlay (not different thicknesses, but different types, i.e. Rubber Waffle, PU Foam, Crumb rubber, Felt) and not had the differences and advantages and disadvantages of the different types available explained, then you should also consider wether the retailer has honestly advised you correctly in your flooring choice as well. Comapnies offering limited choice in such important things as underlay may well be limiting you in the choice and quality of the carpeting and fitting services too.
I had a customer contact me this week 'I've had another quote, its £35 less than yours will you price match?' (this is on a £3000 order!!!!!!) 'certainly, can you confirm the underlay you have been quoted for?' customer 'yes its ******** 9mm pu foam' 'well sir our quote is for Axminster Axfelt65 underlay at £7.99m2 your 'cheaper quote' is for an underlay that we would normally charge £3.99 for!!!!!!!!'
Who in there right mind is 'disonest enough' to claim a £3.99 underlay is a 'premium product' to fit under a £52m2 carpet???? unfortunately there are people out there that try it on and don't care.
Do your research, and as for online underlay prices, do those companies fit the underlay for those prices? OF COURSE THEY DON'T !!!!!!!!! so expect fitters and shops to charge extra for fitting if you supply the underlay.
It's also well known in the trade that some of the online retailers regularly pass off seconds and end of line lower grade products off as premium grade.
In general you pays your money and take your choice, you will have more come back if you purchase everything together and can ofetn negotiate further if you buy all your services from the same place.
If you hav'nt already, seek the advice of a good local independant retailer, they are likely to offer you more time, advice and probably a bettr price and service than the out of town 'sheds'