I've been to a few Spanish wedding now (including my own). The amount of money you give all depends on how well you know the person. When my brother in law got married, we gave him €2500 which is obviously a special case. Other weddings we have gone to (friends, wife's cousins), we have tended to give 150-200 euros between the two of us.
I think it's good practice to give enough money to at least cover the cost of your meal/drink, ideally with a bit extra on top. Our wedding cost 55 euros per head in Andalucia a couple of years ago, but I'm sure Barcelona would be more. I think 100 euros would be about right. Remember, you will be getting a lot of food and a free bar for your money. I think at our wedding there were about 8 courses, and there was a free bar all night.
Hope you enjoy it.