Well, generally a 51% attack doesn't requier expending cost as much as it requires accumulating capital. With PoW, that capital is mining hardware. With PoS, th

Well, generally a 51% attack doesn't requier expending cost as much as it requires accumulating capital. With PoW, that capital is mining hardware. With PoS, that capital is the currency itself. In both PoW and PoS, a double spend could be done in a way that doesn't "cost" the attacker anything. But it does require accumulating so much capital that its hopefully extremely unlikely that any one person or group (even a government) could accumulate that much. An attack would generally substantially decrease the value of the capital accumulated (again, mining hardware or coins) and so there is a cost there. But it isn't the kind of thing where you can say "1 attack costs X dollars and 2 attacks costs 2X dollars", so the value of a particular double spend isn't really that relevant.