subetalodge says: Estimated User Shop Price: 26,999,999 sP
which I don't think is too inaccurate. it might be a tad higher. it's not all too expensive to finish the bobblehead set where that comes from though.
they've been retired awhile but the 2012 bobbles have never been as expensive as the 2010 ones. the 2010 set has the trident which is better than that knocker. if the knocker becomes too expensive, it just isn't all that useful as a weapon.
Bought mine last month for around 30m I think :)
For additional info - I bought mine late last year for about 25mil (it had a 20% discount as far as I remember)

My hubby bought one for me about a month-month and half ago. It cost him 25.4m.
Best of luck to you!