You can lend one hour mining power, costs are about one hour mining profits, thats about 1 million $ I think
How to do it concret, isn't the point, I think. If our arguments are correct, we have showed that mining costs must comparable with transaction value in PoW currencies.
And the simple argumentation is: Lend much coins and double spent it.
Costs are mining costs during attack and profit is the value of the double spended transaction.
If profit is greater the attack would work, doesn't it?