For me I'll pay a small mark-up but no more - 150-200 pounds. Mainly for a bit more personal space, quieter cabin etc.

A lot of variables though, def on 787 as WT is brutal, I'd think harder if it is old product (747, some 777) and I probably just wouldn't on A380 as I find upper deck quite acceptable.

My usual route was LHR-JFK for a good while and in the end I was using AA MCE outbound which was absolutely fine

I also value the possibility of using Avios to upgrade into CW if booking WT+ outright, though less of a no brainer than it was pre-devaluation