Marketing activities have two impacts on price. Firstly the marketing spend can simply increase the price of the product, since obviously that advertising has to be paid for. Secondly, advertisers would argue that by advertising, say, BMWs they sell more BMWs, which results in higher sales and so economies of scale, and therefore a lower overall price.
Obviously if more formula were to be sold, that would cost parents more money, since breastfeeding is free and formula £££, so the second argument, increasing the total sales of infant formula, is not a good one. And the other impact, higher prices, is clearly not what is wanted either.
Frankly the best thing that could be done to reduce costs is generic packaging and banning the promotion of follow-on milks. Marketing spend on follow-on milks is done in order to promote the brand name shared with the infant formula (which they cannot legally). Since follow-on milks are cheaper than first milks, it's clear that consumers of infant formula are paying the cost of this advertising, even though it's supposedly illegal to promote formula.
E.g., this advert:
which makes numerous references to breastmilk and features a young helpless baby who should be getting the majority of his calories from breastmilk and which claims to be promoting this product:
img.tesco.com/Groceries/pi/317/5000378998317/IDShot_225x225.jpg
even though this product sells in much higher volumes and comes in almost identical packaging:
img.tesco.com/Groceries/pi/270/5000378998270/IDShot_225x225.jpg
(but cannot be advertised)
It's like cigarette companies circumventing a ban on cigarette advertising by spending millions of pounds promoting Marlboro candy cigarettes.