BMW i8 vs 840d as a daily

[AlexNJ89]

For the BMW i8 It feels like there's a £40k bubble which is about to burst, the first person to sell one in the £30k's will drop their overall value significantly.

I'm watching the market as plan to buy an i8 in a few months, but also wondering if the new 840d will drop down to the same value and whether my mileage (about 20k per year) would mean I save more over time on an i8 or if the 840d will return similar MPG.

The 840d may hold its value better. New Design language, more practical, potentially similar mpg (maybe!?)...