In part, it depends on which airline we are talking about. IMHO Delta miles are worth less than UAL miles which are worth less than AA miles.

I personally don't like to pay anything extra for miles, 'cause there is heavy risk they may become worthless or heavily devalued. However, I have, a few times, paid one cent a mile or less to get some AA or UAL miles. I don't think miles are worth any more than that.

Of course, working backwards give a different result. 90K miles will take you J to Europe from the West Coast. Such a ticket might cost, at retail, ten grand, thus, if I do the math right, making each mile worth over ten cents. But, I don't think any sensible person would pay anything like that for miles.