You can reverse a block with 25BTC worth of hashing power. That means each block costs around $6000 to reverse.
Not sure where you got this from... but to effectively reverse a block, you need to create a longer chain, faster than the one with the block you want to reverse.
Last time I've checked the global hashrate, 51% of the network costs much more than $6000...
51% isn't even correct, that'd be what on average you'd need to beat a zero confirmed transaction. Even that is assuming you posted the tx around the time the last block was created.