I hadn't noticed this as I don't use slide film but on checking my list of film stockists you are right with almost very stockist charging the same except a company in the case of Provia film called Keyphoto which is selling Provia 100f 35mm at £5.95 which brings it much closer to the 120 price.
If Keyphoto can manage it why not everyone else and it begs the question is it a manufacturer pricing issue or a retailer one when one retailer can do it for a lot less?
I suspect it is a retailer issue. If you check on a range of retailer prices for film, paper etc it has been my experience that each will have relative bargains and each will have relatively expensive items but they aren't the same items at each stockist so Keyphoto might and probably will be beaten by several others on other product lines
The problem with a cherry picking approach for the buyer is that each time there is postage to take into account which may negate the benefits of cherry picking and each stockist is well aware of this.