There are so many possible routes for getting from either Milwaukee or Chicago to Madrid that it’s kind of dizzying. If you don’t think the price alerts are working well for you, then I think you should choose one of two options- start doing searches yourself or get a travel agent (yes they still exist).
If you’re going to start doing searches yourself, I would just plan to visit the websites of Delta, American and United. Hunt for round trip fares on all 3 sites for the two possible departure cities (Milwaukee and Chicago) to Madrid. Then, as trecile says, you’ll get a sense of what the fares are and will be able to jump on a good one when you see it. Fares are totally unpredictable and you never know when a good fare will pop up, but you’ll be much better able to recognize it if you have familiarized yourself with the going rates.
I have a friend who was in a similar situation to yours (not going on a camino, though) and she was totally uninterested in spending hours on the internet looking for fares. She went to AAA, and a travel agent there helped her. I don’t know whether there was a charge, but this agent also booked hotels and rental cars. I don’t know if they would just do the flights, but if you are an AAA member, it’s worth a call.
Just one heads up - I think the strong consensus here on the forum is that you should buy your tickets directly from the airlines, not from a third party like Expedia or Kayak. In case of a problem, cancellation, etc, you are in a much better position if you have a direct relationship with the airlines.
And one other thing — American is a partner with Iberia, which is of course the main Spanish airline. If you fly American or Iberia, you land in a better connected terminal in Madrid, and if you fly American, you can easily add on intra-Spain tickets if you want. For instance, if you want to fly from Santiago to home, it’s easy to do if you’re on an American or Iberia flight. Flight leaves Santiago at about 9, which gets you to Madrid in time for most of the flights to the US.
Good luck with all of this, it’s much more daunting than actually walking a camino!