Also, regarding prices, it’s not simply an objective fact – you are making a point of pointing out that the products in one store are more expensive which will discourage the shopper from shopping there. If you hadn’t pointed it out, she might not have cared enough to look into it. It’s possible that she is someone for whom money is not such an issue, so there was no (halachic) reason to point it out to her.
Furthermore, one of the conditions of toeles is that you have to say things as unnegatively as possible and mitigate the bad by saying any positive things you can. Presumably, if you are telling someone that the merchandise in Store A is more expensive than the merchandise in store B, then you would also have to let them know that it is better quality if it is relevant to the person you are speaking to.
But let’s say you are speaking to Shira who doesn’t know and doesn’t care about quality. All that matters to her is the price. But once you mention that the merchandise in Store A is not as good quality as the merchandise in store B, she will get nervous and think she has to buy in Store B, even though for her it would have made sense to save money and shop in store A. So you have just spoken l’H about Store B for no reason. (and maybe caused shalom bayis problems when her husband gets mad at her for spending money they don’t have).
On the other hand, Tova doesn’t really need to know which is less expensive – she needs to know which is better quality, so there was no reason to point out to her that Store A is cheaper. All she needed to know was that Store B is better quality. But if you tell her that Store A is cheaper without mentioning that Store B has better merchandise, then she will feel she has to shop in Store A. Since for her the quality of the merchandise is what matters and not the price, it was l”h to specifically point out to her that Store A is cheaper. And now she will have shalom bayis problems because her husband would have preferred that she buy the better quality merchandise.