How long is a piece of string?
I don't know about requirements for non-EU citizens wishing to live here but as for general living costs, it depends entirely on your tastes and expectations.
WE don't put any burden on the system other than our reciprocal entitlements as EU citizens but you would have to consider private health insurance as an essential addition to your normal living expenses.
How much it will cost depends on where you choose to live. Some areas are cheaper than others - and there's usually a reason for that. I would say that the cost of living generally is slightly higher overall than in the U.S.
The cost of renting varies from 300 euros monthly in the northern coastal regions and inland areas to the sky's the limit in the Marbella area.
We have assets in Spain and in offshore banks - it doesn't matter where your millions are kept!