Just like the unattainable mpg figures my car manufacture claims my car will do.
Gatwick's cost £110M 10 years ago including 11 additional stands which was supposed to add 3M passengers
That was built to allow 747-400's to pass underneath. You can be sure that Luton would get a cut down price version in the style of the 150m of footbridges Luton is getting for its car parks.
As for a potential crossing point I would say it would be cheaper to build off the existing pier as there is only a single taxiway to cross. It would have to be built at an angle though to avoid Harrods apron. If it came off yet to be built pier B it would have 2 taxiways to cross including new taxiway Foxtrot and would end up being longer than Gatwick's.
Whether the airport would want to sacrifice its long term car is debatable as that is a big money earner for the airport and another location for it could be hard to find.