So far it seems like if MS did raise the price they would lose half their membership so that would hit them really hard in the wallet.
Not really. Microsoft wants to raise its price in order to maximise its profits.
There are a couple of things going on in the equation:
1) Net growth of xbox live subscription
2) Infrastructure (e.g. servers)
Microsoft wants to maximise the use of its current Xbox live resources so the simplest way to do this is by price control - i.e. If every server can only handle 500 gamers & there are 800 people who want to play, the simplest solution would be to just increase the price enough such that you will have only 500 gamers left who are willing to pay the increased price.
It is much better than channeling microsoft money into upgrading the servers to account for the 200 gamers.
The point is, the small increase in price isn't due to "upgrades to servers & increased cost of maintenance" as Microsoft would have you believe. It just a form of price control.
e.g. Laggy servers due to too many gamers? Simple, just increase price to get an optimum number of players & instant improvement in server quality.
Because of this, you'll often have an intermediary period just before they actually add more physical servers where the quality of Live would be quite poor because they are really trying to stretch the resources before making the switch.