I'd say my upper limit for a cash fare in J is roughly $3k round trip or about 90k miles each way. It depends on the airline, the routing, and how badly I want the flight. Personally I have no need for some of the more extravagant experiences in premium cabins that appeal to others. For me, all I care about is having a lie flat since I cannot sleep sitting up and don't want to be a zombie waiting for bed time at my destination. I'd be happy with the Y meal if it saved me decent money or miles on the fare. I also don't go crazy on the booze, sans a glass or two of red wine to help put me to sleep after dinner. I do think there's an untapped market for lower cost Zipair-like, bare bones lie flats internationally, especially between the US and Europe.

My most recent international J data points in the last couple years - YVR-BNE on AC was paid with points (71k I think?). FJ SYD-NAN-LAX was 55k Alaska miles. LATAM LAX-SCL was $2500 or so RT with AMEX IAP. I don't fly internationally as often as many here, but when I do, I try to do it in business.