There are actually three. Vat pasteurization is the oldest, it involves holding milk at 145 deg. F for half an hour. High temperature, short time (HTST) pasteurization involves bringing milk to 161 deg. F for 16 seconds. And Ultra-pasteurization, or the ultra high temperature (UHT) process, involves bringing the milk to 280 deg. F for 2 seconds.

Time is money, so vat pasteurized milk is more expensive than HTST milk which is more expensive than UHT milk. The downside of higher temperatures is that more of the flavor compounds tend to be destroyed in the process. This has a secondary effect of driving the milk producer towards lower milk quality, since it all tastes the same after the UHT process.

I'm not sure, I always assumed UHT was a bit cheaper but never paid attention (or if I ever did look at the difference I've forgotten).

I just had a quick look at a UK supermarket's online site and both seemed similarly priced with some comparable options being cheaper on one side or the other. Without bothering to spend more time actually doing the maths, and looking at more than one store, it could be that there is a significant difference on average.

In France/Belgium I equally never bothered to check the price difference (I'd still be choosing pasteurised where available if it cost 5x as much), but anecdotally I saw it more in higher-end (most expensive) supermarket chains.