It's impossible to give a figure for a completed car, because 1) what with the wait for a kit, plus the time it takes to build it, any costs will only rise in the meantime; and 2) whatever spec you decide on now, you'll change (look at Leigh's build - he could have probably bought an original Stratos for less than he's spent so far, and he's not finished yet...)
What Jim said, about Autocar 6 years ago saying "build one for £30K", was probably not far off correct, but things have changed massively since. Alfa V6 donor cars have risen dramatically, the parts to rebuild said motors aren't cheap, LB's prices have risen (they had to, they were too cheap), their trimmers are more expensive (but much, much better), etc. etc... A current LB can easily be taken for a proper production car, and that's as it should be, because it's a serious motor, which (along with the Hawk) is starting to be taken seriously by moneyed classic car enthusiasts - look at Nick's blue car at Stoneleigh, which sold for well over £50K.

My advice would be to decide what sort of car you want to build (2.5 Alfa, up to supercharged Exige), paint or not, blah blah blah, then speak to Craig or Lydia who'll give you the current state of the game. £45K? Maybe, just maybe, but I'd allow another 5 to 10K on top to make sure it's a car you'll be happy with - no point doing it as cheap as poss and then being miserable cos what you really wanted was one with... you get the picture!

I'm not saying what mine will likely come out at when it's finished, but let's just say it's already more than I've ever paid for a car...but I was determined to do it the way I wanted, and do it right. Also as Jim says, it'll be worth, and continue to be worth, a shedload of moolah as long as originals are priced in the, er...stratosphere... ...so not a bad place to keep your dosh. So have fun speccing it out, then get the word from The Peeps at the Factory