I agree with Sean. Any type of price is based on good old fashioned supply/demand and it's hard to gauge what a fair price is without knowing what the demand is. Also, if you give away the product/service and you can get tons of people to use it, there will be revenue opportunities other than sales, such as ads.
I worked at a company where we gave away 60 minutes of the service each month, in the hopes this would get people to pay for additional hours. People would use 59 minutes and 59 seconds of the free service and then stop each and every month. This proved that people liked using the service but didn't want to pay for it. This was invaluable information as we knew the product was good enough for people to use it and they liked it, but they didn't want to pay for it.
If we only had a paid option to use the service, we wouldn't have known the service was good, but maybe priced a bit too high. High price is MUCH easier to fix.
First thing you need to find out is if people like your product/service. By having price in the mix, you'll never be sure if people don't like the service, or don't want to pay for it. Make it free to begin with and you'll find out for sure if people like your offering at all. Then move on from there.