Pipeline product with Tiers

I have to create a pipeline product with the following configurations:

0- 1Gb  ----  Rs2/Mb
1Gb-2Gb ----  Rs3/Mb
2Gb+    ----  Rs4/Mb

If there are 2 session of say 1.2GB, 1 Gb. I would like it to be rated in the following fashion:

For 1.2GB session :: Upto 1 Gb, it should be rated with Rs2/Mb and the rest of 200 Mb should be rated with the second tier i.e Rs3/Mb.

For the 1GB session :: Since 200 MB of the 2nd tier is already over due to usage from the previous session, the first 800 MB must be rated at Rs3/Mb and the rest of 200 Mb should be rated from the next tier ie at Rs4/MB