It will use more battery, you need to experiment with that. It may very between phones though. Leaving the display backlight active (showing the data continuously) whilst it’s going (this is an option) will be the biggest drain though. I find it’s perfectly ok for a ride if the battery is charged beforehand. Problem is without the backlight, many phones have an almost black screen.
As far as data is concerned, the data rate is fairly low so I’d not expect it to run away with your cash too much. I have checked, and the data file sizes mine downloaded whilst live tracking look to be about 20Kb for half an hour ride, so work that out at your GPRS rates. (Plus some for overhead). GPRS rates vary from about £4 per Mb down to 20p (Asda Mobile 🙂
The Sportstracker app which runs on the phone is neat and works very well. My gripe is that the Beta software which runs on the web machine at the observer end is pretty clunky when run on a mobile phone. So you can report your position pretty well to someone sitting at a PC, but if you are mobile yourself, and want to look up the Sportstracker web site using your phone to check on someone else, that is not slick.
GPS in mobile phones is nowadays “network assisted” if you enable that option on your phone. Doing so helps enormously. This enables most of the coarse aquisition code (basically timing data) for GPS to come from the cellular network rather than the GPS satelite. This greatly improves GPS reception, and appears to allow operation in really GPS areas. My N78 will pick up half a dozen satellites even from inside my house, the Garmin strugles to do that.
Take a look at the Google Latitude also. I ran that up the other day and it’s very neat. It gives no logging, but is ultra easy to show people where you are. It definitely was much more heavy on data than the Sportstracker though because it runs in mobile google Maps. So it downloads maps as it runs as well as reporting your position. I think once they are downloaded they get saved so the data usage for that bit should peak out after a few MB, at least for a local area. Nice thing is the “observer” can be mobile as well using another phone.