In short, I'd say there's two reasons 4SRKT why tours are cheaper over in Ireland. It is not as big a hobby over here, nowhere near as fact. The RPSI in the main cater for the day out brigade, families etc who want to experience a steam train. Therefore, no way would they get away with charging ridiculous prices.
Secondly, the RPSI and NIR (or IE in the case of Republic) are the only two bodies involved over here. RPSI approach the the railway operator concerned and request to run a train on a certain date. Drivers, timings etc is discused between the two.
Now look at the UK, you have the tour promoter, Network Rail, TOC, traction & rolling stock provider plus God knows who else in order to run a tour. All this adds to cost, plus with the likes of Chingfinder, they're trying to make a profit. With the RPSI in the main they want to run a tour to cover their costs, and perhaps rely from onboard sales to bolster the viability and put any profit back into the charity or future tours.
I'm not into kettles in the UK anyway, but I will say even if I was, there would be absoloute no way I would pay that sort of money for what is technically a brand new loco. If it was something like Mallard that had been returned to the mainline, I might and just might because of the loco it is consider parting with such money. Tornado to me is the equivalent of doing a 66 or a 201 on a railtour, and that is never.