I reared 2 lambs on goats milk this last year, started off on powdered by others and I had to deal with scouring until I gradually moved them onto goats milk. They did fine after that.
I seem to remember the milk was SCA Shepherdess £23 per 10kg (spring 2012)
I can't see it being worth buying goats milk, having goats and using the surplus milk is a different thing.
It all costs, the bottles seem expensive, washing up, keeping bottles clean and sterilized regular,
Then when they are grown up they break the gate down if they think you have a bottle for them!
I wouldn't put a lamb direct onto the goat, I've read and been told how lambs are much rougher than kids (mind you, watching the kids feeding last year did make me flinch now and then

). Also I believe they can give the goat infections such as orf.