I won't say a cost, as everywhere charges different. I would say budget on 20 hours at your normal instructional rate plus test (course length is 15) and that way you won't get hit with any surprises.
When you are booking lessons however it's the sort of thing you want to do in a relatively short length of time, to concentrate the learning. If you drag it out over a period of time you could find it takes you longer, as you'll be constantly going back to recap things.
You'll find it improves your general standard of flying - as well as giving you a better background from which to make
informed comments about IMC flying
