a lot
prob around £1000
And the rest, our 5x2x1.5 cost us £500 in rock alone then add on an RO unit, protein skimmer, powerheads, lights, salt, a refractometre etc.
For a tank as big as an 8ft you'd probably need a sump as well or water changes will be an absolute nightmare.
Ours was a basic set up and I reckon we probably spent around £1800-£2000 not including fish.