Each blockreward is 25BTC=$11,000
It would cost at least $55,000 for someone to unspend a txs that is buried 5 blocks deep.
Yeah, it doesn't work that way.
It's not like the cost of mining equipment is equal to the cost of 1 block,
in any way, shape, or form.
Unless you have 51% of the network, the odds of you building
the longest chain drop exponentially the more blocks you try
to unspend.