I'm with you here. I got my complete Overlord for $4, but that is no more a true reflection of what's it's worth than a $30 complete Chromedome is.
To make this thread work, there has to be some objectivity. I think reframing the question for each person as:
"if I were currently looking to buy this figure now, how much would I pay for it?"
But even then, there are inherent difficulties because each person wants their figures in different conditions, each will accept figures in different conditions and each places differing values on particular parts. For example liege may pay more for a complete Siren so he can get the right head laser and complete his siren but the next guy would be happy for a Siren that is missing his right head laser, thereby paying less than what liegeprime would pay, and wait and see to get it someday (little knowing he/she might wait years before obtaining it and hence pay a higher price later).
I'd say though, if you want it, it will make you happy, then it's worth whatever the price you pay for it. What it's worth is a question of value not cost (where value is the importance you place on it versus cost which is the money amount you paid to get it).