My finances work out like this:
Cost of IMC rating: £2200 (varies from 2000 to 2400).
I fly an Archer III for £65 per hour. 15 hours: £975
This gives a difference of £1225.
Now factor in A/C hire for test (£200 ish), test fee (£150 ish) and issue fee (£150 ish). By the time you have the rating in your brown book, I'd be talking more than £1700 extra for the rating.
However, you rightly point out that there could be hours of saving on the IR as a result.
I reckon that, best case scenario, it would cost me over a grand. More likely closer to two.
This seems like a drop in the ocean for flying, but it is actually a significant amount of money.
I'll see what's in the bank come 120 hours of logged time.
EK