While i can not answer your question i am interested why one earth you would want to ? When i lose a 3g signal and try to get online it so painfully slow i normally just give up and wait until 3g back again

 

Go to settings -> wireless and networks -> mobile networks -> Network mode

By default it should be set to GSM/WCDMA (Auto mode)
Change it to GSM only for 2G mode or WCDMA for 3G mode.

Hope that helps

 

Make sense

 

Though I have answered your query you should really ask your network provider on their billing process.

If you have a 2G pack installed then even if you are connected over WCDMA they should charge you the same.
And if you have NO data pack then even if you connect over GSM any data transfer that takes place will be billed to you.

If you wish to just disable data transfers when you do not have a internet pack you should just long press the power button and Disable Data Network Mode from the pop up menu.

Even I have a 2G data plan enabled and I stay on the GSM/WCDMA mode and am not billed extra.

Hope this helps.