How much you should bring all depends on how much you want to do.
If you're going to check out museums or other things lead by tour guides $50 euro will get you by & cover meals.
If you're just doing walk about stuff, bring enough money to feed yourself for the day and some extra incase you see something you want to buy.
Also budget some money for public transit. I've never been to Spain, but I assume they have something where you can buy a pass for the whole day. That'll probably run you about $8-10