Here in Italy there are a lot of rate plants depending by your vaforite operators. So ... exactly your data/month use depends on you.
The only thing that I can add is that in N900 you can configure as you want your apn and internet connection, so you can use also a special promotion made by the operators for only one mobile.

Example: Here in Italy you can ave 2 Gb/month ONLY if you have an Iphone. But ... the same SIM works great also into the N900 :-)