I think it depends on whether you want it to be a van-shape or a box-shape. The van-shape conversions seem to be cheaper as less work needs to be done (and presumably they need fewer materials). I used to have a van-shape and travelled one 16.2 horse very easily - it wasn't big enough for my 17hh so had to sell it. You can buy good conversions to a van-shape for about £9000 (with perhaps a 7 year old van with 100k miles) so I guess the cost of conversion would be £9000 minus the value of a van. Plenty of people on here will say that they won't travel anything bigger than a sheep in one of these... so if you agree with this, you might want a box shape, which seem to cost more like £12k and up for a new conversion including the cost of the van.
