I like the URL idea - ID numbers are not very friendly (and as you said are bad for privacy).
I'd like to keep pricing simple - pay a fixed price and use it as much as you like. This to me is both easier to implement and will encourage people to use it more.
But about the pricing; consider this:
* You'd probably earn hellofalot more with individual pricing? I mean, I pay per sent SMS/Email/Whatever on other services. It's the usage that will cost you money in terms of data-transfers?
If I'd use your service with a newsletter of 100 000 subscribers, let's say 10% would answer my very quick survey, you'd have to host these 10 000 users throughout the survey with $26 in your pocket.
It might be sustainable in the long run if you have enough users that does not utilize the service that much - but with a couple of enterprise users you might be out of luck(?)...
I'd still consider thinking twice about your price-structure, you might want to charge 24/month, including X answers or something?
I'd certainly be willing to pay a fixed price per month (Maybe lower) and then $0.xx per answer to my survey?
Would you consider charging for each Survey or each Response instead?