So I think 50% of new-price (29.99 USD) in BTC is probably a fair starting bid. What do you think?
Yes - according to Mt.Gox they only work with Mt.Gox - they also offer "unbranded ones", but I don't know if there is a way around then or how they are technically locked to Mt.Gox.
'unbranded' just means no logo on it.
You can only use them on Mt. Gox, because they will not disclose the 'shared secret', that is the code from which (together with the Unix epoch time stamp) the one-time-passwords for verification are computed. This is not to annoy the customers - not disclosing the 'shared secret' makes cloning the yubikey impossible, increasing account security.