Your account is worth what someone is willing to pay for it. Not a penny more nor a penny less.
exactly this, also there is lot of variables that you should count in, like the quality of post that your account already have, and things like that, this is why price depends on how many someone is willing to pay for it, i totally agree with you.
BTW pay for a Full Member account is near strange for me since be a Full Member will not take you so long, but ofc people do what they want
As i'm seeing 0.04-0.07 BTC should be fair enough for that rank.