I'm not sure that's true. Typically (and of course other jurisdictions vary anyway) there doesn't have to be money; there just has to be a consideration, which can be money. Something of value flowing both ways. A lot of contracts end up with a consideration on one party of something nominal like a dollar or a pound, but it doesn't have to be money.

"You get my details and my attention and can harvest my data and make money from it" certainly sounds like it could be argued as having value.