Is there any reason that the ultimate price for these services won't approach $0? They're not actually providing promotion, right? That seems like the only real value they could really provide.
They do promote. Many people browse the site, receive emails, etc. Kickstarter makes a point to put in the creator's dashboard a pie chart that shows how much their promotion has brought in vs. other means. In my experience their slice is between 25-50% (skewing smaller as projects are more popular).
I find out about 90% of these things from the kickstarter website or emails saying my friends invested. So I would say for the projects I funded, I would have never given them money if they weren't on kickstarter.