After walking back from the local shops and thinking about how the cost of a "free" plastic bag must be covered by greater (if only marginally) costs in the shop as a whole, I decided to look around the web for some numbers on how much companies spend on advertising; figuring that the costs of advertising must eventually be born by the consumer (i.e. we indirectly pay to get advertised to). In particular, since generic brands are so much cheaper, I was looking for figures on Coca Cola. It turns out that in 2006, on revenues of $24 billion, a full $2.5 billion was spent on advertising by Coca Cola. Very roughly speaking, then, 10 cents of every $1 spent on a Coca Cola product goes on paying for the advertising that was meant to persuade you to buy the product in the first place. Frankly, I'd prefer to make up my own mind and pay less, but ho hum.
There are wider consequences to the average consumer paying for advertising. The majority of media services are heavily reliant on advertising revenue, even to the extent that they are willing to offer content for "free". So services that we consider "free" - Google, Facebook, Blogger - are indirectly funded by the average consumer buying advertised products - the products have a premium price that allows the companies to spend money on advertising that then funds companies offering free services. Basically, "free" services are paid for by us spending money that indirectly fund campaigns that are meant to persuade us to buy more products. "Free" products, and the majority of the internet, then, is the outgrowth of relentless (and non-socially-productive) mass consumerism, which is kinda depressing for anyone who laments the increasing consumerisation of the world but uses these services.
Anyhow, in the process of looking for this information I came across this website, which deals with the politics of food - http://www.foodpolitics.com/ - seems well worth a look for anyone interested.