No, nowhere near £100 (I never mentioned £100, I don't think there has ever been an Amiga RTG solution for that price) especially as it will probably need an accelerator to hang off.
The bus on the A1200 will certainly limit it (is it zorro II?, I can't remember). However, if you look up the Raspberry Pi, you'll see the such chips are dirt cheap even in small amounts.
The circuitry needed to interface it would be fairly pricey I should imagine. also, a lot of these chips rely on binary blob drivers, making them all but impossible to port.
There was an RTG card for the A1200, but I can't remember any reviews about the speed.