if that account has good reputation and trust the price can be much higher,I have seen hero members sold at 0.6-1 btc here.
This account is bringing 0.14 btc per month (provided it doesn't get kicked from the sig campaign) so it can be considered as an investment with 4-5 months return,so yeah like gentlemand said-
start an auction and see where does it get you
another thing to consider ,as it affects the price ,is the btc address connected to this account+e-mail
If you care for the other members on this forum then you won't sell the private key with account.
I don't think someone who buy an account to earn someone from signature campaigns really need private key..