Like everything else, it depends on how important the cost is to you. It will vary person to person. 20k miles to upgrade? If you have 30k miles then you have to decide if wiping out 2/3 of your account is worth it. Or should you save and keep piling up for other uses. For me, I've got 400k+ miles and have a hard time using them on anything but upgrades. So it's less expensive from my point of view.

One thing for sure, you will never justify business or first from a pure $ point of view. The money it costs, whether cash or miles, doesn't justify a more comfortable seat for a few hours. Accountants will always tell you not to do it. Then you have to decide if it's worth it to you.