Now that can be enough to make a decent living, have your own flat etc. but it's not, "let's go on holiday to the other side of the world twice a year" sort of money either.
Also there is much less a culture of senior developers, the idea being that to progress in your career you need to stop coding and start being a manager (not true everywhere but the norm)
Yeah, that's why salaries are so low.
What's Messi make again?
Thus, you should bring to the interviews that have experience working in USA. I think that is pretty valued in Spain.
On the other hand, Madrid seems to have an American expat community, there are some meetups [4][5] and facebook groups, etc.
Don't know about Barcelona.
Good luck!
[1] Take a look at https://www.siliconrepublic.com/start-ups/madrid-spain-start... and https://www.seedtable.com/startups-madrid
[2] https://www.glassdoor.com/Job/jobs.htm?suggestCount=0&sugges...
[3] I know you want to work for a startup but Amazon is in Madrid, maybe you are interested in that?
[4] https://www.meetup.com/es-ES/expats-and-americans-madrid/
[5] https://www.internations.org/madrid-expats/americans
EDIT: formatting.
What's your quality of life currently in U.S.?