You really can't compare the American and Spanish economies on cost of living alone because American incomes are also much higher. For instance, developer jobs here in the UK that aren't leadership roles top out around 60k outside London. So yes you could probably say that Spain is poorer than the US 25 years ago if you only count individual income.
I agree that you can't directly compare but there are some aspects that you can: cost to travel, cost of common goods, etc... Another two aspects that can be compared are the absolute amount of investable income after cost of living and the taxation rate on that.
The rough measure is that you really want those Silicon Valley tech incomes because the investable income is so great. You can value that just via the market actions (how many people are willing to move if given the chance to).
