What value do you assign the miles you use for the J seats? Assuming you won't pay full J price, what WOULD you pay if it was an auction?

The answer depends a LOT on what else you would use the miles for, if you have a bottomless pit of miles or a limited pool, and what value you give to rumors of SM changes and your future flying habits. Also what the domestic trip was going to cost and if I needed the MQMs for anything that I would be giving up.

There is no one-size fits all answer. You have to do the math to match your situation.

Personally, domestic personal trips are almost never last-minute, and I'm paying 5-7 cpm (per MQM) which is into MR territory. If you get bonus miles and need them for your next J ticket, that's another personal variable.