Rough cost to replace vented hot water system with non-vented

I have had widely differing quotes for this - I'd like to know roughly what the expected cost would be to take out a vented hot water cylinder from the airing cupboard, replace it with a non-vented one in our old boiler room (now empty) which already has a soil pipe to ground for the safety outflow. This would also include removing two redundant header tanks from the loft, and two power-shower pumps (as I understand you can't have those with a non-vented system). I appreciate you can't be precise without seeing the job, but a ball-park figure would be appreciated. Thanks.