Barry,
If you build a kit it will need to pass the SVA type approval test, which requires all sorts of deviations from the real Cobra specification. If buying a built or partially finished kit, make sure it complies with the regulations so it can be registered, or it could cost you a fair bit of money to make it comply.
AC CRS Cobras usually sell in the £40k to £50k range; AC MkIVs perhaps double that, and Lightweight MkIVs perhaps treble.
Finished Hawk replicas possibly in the £25k to £40k range.
Other kits from £5k to £50k, excluding the exact replicas such as Kirkhams.
Hope that helps. Good luck!
These are all rough guide prices, but give you an idea.