Subscription TV is a very different model to commercial TV, which means direct comparisons arent that straightforward.
FOM gets paid an amount of money from a broadcaster, and the broadcaster then has to make that money back. For commercial TV this will be through advertising during the show so they are dependant on having enough viewers watch the show in order to charge enough for the advertising. This is quite difficult when broadcasters like C4 aren't advertising during the race.
Sky on the other hand get a sizeable chunk of the funding for their programming from the subscription of viewers. They also sell advertising but they are less dependant on this. In another thread I think we worked out there were about 5m Sky Sports subscriptions in the UK. Sky don't really care if you actually watch or not.
So in terms of viewers free to air gets about 20m annual viewers, whereas Sky is more like 4m. However in terms of whos 'funding' the broadcaster, free to air is 20m vierers while Sky is actually 100m subscribers (20 races x 5m subscriptions) if measured on the same basis. When you look at it like that you can understand why the Sky deal is rumoured to be about £1bn over the 6 years (£166m a year, whereas Channel 4 currently pay 'just' £24m a year).
There was a blog a guy did a couple of days back that showed that in theory (there was a lot of speculation, but based on solid foundations) the Sky deal (which is only a change relating to just 5% of the TV audience remember) would be worth between £2m and £10m to each team in terms of prize money. That more than makes up for any potential reduction in sponsor income.
This deal, while crap for us fans, actually works out to be a net benefit to the teams.