Why are y'all looking at the total ticket price to determine the value? That's completely nonsensical.
If you get a free ticket and then are asked $500 to upgrade the only question is whether the upgrade is worth $500 to you. It's the same upgrade as if you paid $1,000 and then are offered an upgrade for $500. The fact that the total fare is $500 in the first case and $1,500 in the second case is irrelevant. If $500 is too much for the upgrade then it's too much for both cases. If it's worth it then it's worth it in both cases.
By your logic, would you take a $500 upgrade on a free ticket but turn down a free upgrade on a $1,000 ticket?