An unfortunate part of experimenting with advertising is that it's expensive. Trying to find the right audience for the ad is very costly, to the point where I don't understand how advertisers can afford it.

I say this as a tinfoil-hat person: narrowing the audience is going to need a lot more info about the person. Particularly, their browsing history (i.e. remarketing). Knowing this would raise the conversion ratio immensely at the cost of privacy.