I am looking for some study/comparison on how battery-consuming are specific services/sensors (3G, Wifi, GPS, Bluetooth, accelerometer) in the iOS devices.
Has anyone stumbled across something like that?
Unfortunately there is no simple answer to your question. Battery life obviously depends on what is running and what your phone is doing when you are using it and when your not using it.
Hopefully this list will help you out:
3G uses more battery life than Edge. The lower your signal, the more your phone has to work, thus using more battery, in order to keep that signal. If you have poor signal, phone calls/internet usage will drain slightly faster than on a good signal.
Wifi uses less battery than 3G, but still an ok amount. Signal issues can cause more usage like in 3g cases.
Wifi and 3G are still used while your phone isn't being used to access Mail if you have Mail Fetching turned on. A lot of times people who have bad battery life are always fetching their mail every 15 minutes, even if there is no mail to check, it still does this. I recommend turing that off or switching to a Push Email Account like Gmail or Mobile Me.
Also, Exchange Emails can sometimes get corrupted on your phone and drain battery life like crazy by constantly syncing and checking for mail over and over again. Removing and re-adding the Exchange email has shown to fix issues with Battery Life in certain cases.
Also, close apps that are not being used anymore in your Multitasking App Tray. A lot of times having 40 to 50 apps in there can cause a bit more battery drain. Now this doesn't mean clear it every time you use an App, just every once in a while.
Same thing goes for restarting your iPhone/iDevice. Every couple of weeks or so powering it off and then powering it on can clear up little things that could have been using slightly more battery life.
Bluetooth 2.1 Devices use a lot of battery. Bluetooth 2.1, is one of the biggest killers of battery life. If you do not use Bluetooth, turn it off now.
Bluetooth 4 Devices are considerably better at battery life, since BT 4.0 has a better low powered mode, but the problem is still with 2.1 devices. I can have BT 4.0 device but if it is paired with a 2.1 device, it will consume more battery life.
Accelerometer has never really come up ever as causing battery life issues or for draining more battery life than other things. I wouldn't really worry about it. Same thing goes for the compass inside the iPhone and Gyroscope.
GPS does use a good amount of battery, relatively between wifi and 3G standards, especially if you are constantly accessing it. Still, you usually use this in a car, and if you are driving great distances then I would recommend getting a car charger anyway. You can get them cheap and they are always useful.
Screen Brightness, the unrealized killer of battery life. Always make sure your screen is at an optimal brightness, with Auto Brightness (energy saving) settings always on in the screen brightness section of the Preferences App. This setting helps a lot. I would also recommend having your brightness at 50 percent. It may seem really dim at first, but your eye will get used to it and it can save a few percentages.
I know its not the most perfect answer and doesn't give you extensive research study related material, but hopefully it will shed some light on the things you wanted to know. I have a lot of experience with different battery issues and fixes when it comes to iPhones as a former Apple Employee.
I hope it helps you out.