A semi-related question to this that we're struggling is: how much should we bother incentivizing those who invite the most with cash, an iPad, or even a contribution to their favorite charity? At the end of the day, this sign-up isn't really directly for people who are going to purchase our service in the future. It's really just a way for people to say "hey, I'm curious - let me know more". We're struggling to determine how much that is really worth.
I wouldn't encourage people to take actions with a physical reward as their motivation.
First, it gets you the wrong type of users, because people completely uninterested in your service will get others to sign up just because there is something in it for them (or create fake accounts, bots, etc...). Second, if you use this approach it's hard to gauge real interest in your service. It should be interesting enough that people will want to show their friends with no reward, if it's not I'd solve that problem first.
If you feel you must incent them in some way, maybe a free month of the service, a "Founders" badge or something else that relates to your service but is intangible might be a better route.
A semi-related question to this that we're struggling is: how much should we bother incentivizing those who invite the most with cash, an iPad, or even a contribution to their favorite charity? At the end of the day, this sign-up isn't really directly for people who are going to purchase our service in the future. It's really just a way for people to say "hey, I'm curious - let me know more". We're struggling to determine how much that is really worth.