I'm guessing the new lens will cost about 50% of the 100-500's price for about 85-90% of the performance.
Let's hope, you are right.
When you
compare (TdP) the EF70-300II non-L to the EF100-400LII there is more than just 15 % gain in IQ (IMO, let alone the 100 mm additional FL).
Pricing is always so irrational when chasing higher increments in quality. ...
The prices are really painful, true.
But I wouldn't call it irrational.
It is the same with other extreme technology (High End PCs, HiFi, cars, bikes, watches etc.):
In the beginning, let's call it consumer or mainstream level you have an almost linear rise in performance per price.
And you have high sales numbers, so R&D costs and costs for the production lines and production process are proportioned between much lesser item numbers.
And because of this you get an asymptotic behavior to a horizontal line in performance per price.
The efforts get much, much higher per 1% performance gain.
The only question is, how much performance do you - really - need. Not how much you want
