First thing is, don't mix up the bank tax with maintenance fees. They're different things, though they both affect you. The financial transaction tax is something else entirely and depends on the type of operation; what you're probably worried about are the commissions your bank charges for keeping the account open. Those definitely vary a lot depending on the bank and the type of account you have.
As for Santander specifically, I can't give you exact figures because they change depending on the product and conditions, but the best way to find out is to go into your online banking and look at the commissions document or contact your account manager directly. What I will tell you is that a lot of banks have been raising these charges, but they also offer ways to avoid them: if you set up direct deposit for your salary, you have a minimum number of transactions or you maintain a certain average balance, they often waive the commission. With 15,000 euros saved you can definitely negotiate, especially if you've been a customer for a while.
If you're really concerned about it, it's worth comparing with other options. Traditional online banks usually have lower commissions or even none at all, though you lose the option of going to a physical branch. If you'd rather have an office nearby, call Santander and ask directly what options you have to cut costs based on your profile. It doesn't hurt to ask, and lots of times they have products with no commission if you meet certain requirements.