I used to have T-Mobile and there were no additional charges in Europe for data and only a small cost for calls. Now we have ATT (since we moved to a location where there is no T-Mobile service) and it was about $250 per month total for both my husband and I to keep our home service while in Spain last summer. For one person/phone $150 per month, but with the second line it was slightly less. It is so much per day up to a cap each month.
Depending on where you start your Camino, if you want a Spanish card you have to wait until Pamplona (if you fly into France). That is the first big city where Sim cards would be available. We always fly into Madrid now so it is easier to get a Spanish sim card. as
@Rick M mentions, there are limitations with all the plans. You can read through the threads and see all the variations.