It depends, In some cons I've made barely enough to cover costs, and in others I made up to 200 euros in two days, but I don't go to very big cons anyways. I'ts not a lot, but i know people who make over 1000 euros at big cons.
My recommendation is that you don't make a lot of copies.
I go with arround four to six copies of the prints because they don't sell very well and are relatively expensive to print.
Stickers are really cheap so I make a fair ammount , maybe two pages of each design, and i cut some of them in the stand to pass time.
Acrylic keychains sell very well but are generaly expensive to make, about that, if you are confident of your designs, I'd maybe make like ten of each and I'd try to ask other people to make a group order to cheapen the shipping.
Maybe you know but there is a spanish artist alley facebook group, I'ts great to ask stuff like this, and it's always good to hear opinions from more experienced people. It's a private group but if you are interested on joining, drop a contact.