Looks like I have a bit of an issue, my roll bar isn't tall enough

. I guess thats the problem with being 6'6" eh....
I sit about 880mm from my arse to the top of my head. From the top of the build table to the top of the roll bar is 950mm, but then you need to take into consideration that theres the bottom rail which on top of that, which is 25mm, the seat supports + runners + the seat on top of that, I'd also be wearing a helmet and if i were to roll the car then i would be hanging from the harness.
So the question is....how much will a full cage set me back?

. I was planning on going for a full cage after i finished the roadster, but it turns out that the CDS tube i'd be buying to support the roll bar will cost me £60.
What's everyone's thoughts?