I think it's about the same cost as learning to drive an HGV so you are almost as well off doing the HGV test instead. My sister did it a couple of years ago and it's approx £1000 all in. So, not cheap!
I paid about £1200 to do my HGV last year, it wasn't much less to do the 7.5t. I went for the hgv as I thought I may as well pay the extra and not have to worry about what I drive in the future.