In a nutshell....

£250 for the hardware key & software.

£80 - £180 for a lambda sensor depending on options.

£20 for teh stainless steel boss for teh exhaust.

£20 to weld it into teh downpipe if you dont want to do it prior to teh visit.

£50 to wire the lambda into the management system to get the signal and heaters working.

This will give you total closed loop fuel control.
Bear in mind i dont do the sofware off teh shelf for any larger than 803s. If you want 400s or larger i will ONLY live map it.