If Bitcoin were to go the Proof of Stake option (using sidechains perhaps), one of the problem people have with the idea is that it will become much less secure because it would be very cheap to perform a 51% attack.

Some PoS coins boast that you need more than 51% of the coin to control/destroy the normal functioning of the network, but we'll just focus on 51% now.

How much would it cost to buy 51% of BTC (not including lost/unused/unallocated)

Please comment below.  

The reason it is 51% of "owned" coins is that in PoS only actively owned/used coins count, so lost/unused coins don't count... at present, i think there are about 10-12million BTC that are "used"... so the question is, how much to buy about 6million btc...although this figure may change if the purchase happens slowly, over a year or two. The cost estimate still stays valid/interesting