Not sure if my thinking is simplistic ...
When you look at the course costs at the minute, there are two prices, one for 'domestic/home' students, and one for foreign (which is higher). This is explained on the OU site by saying that the government subsidises courses for UK students, but foreign must pay in full.
So logically, to know what any particular course will cost when the subsidy is cancelled, just look at the foreign cost now, and that's what we'll pay in the future (as foreigners in our own country!).
Work out which courses have the most subsidy, and do them before all this happens.