Travel agent? What is that???
The internet has almost put them out of business! If you have the time, you can find all the information you need on the internet; tell the agent, if you use one, what you want after some research.
I have found that one-way flights from and to the U.S. are quite expensive, sometimes each one costing more than a round trip. Even if you are spending 70E per day to stay in Spain, you could stay an extra 10 days for about $766 at the current exchange rate. I suggest booking a round trip ticket before you leave, allowing enough time to walk where you want. Some airlines allow you to change your reservation for about $250, but read the fine print. Airlines are a duplicit0us bunch of bandits, and an actual change may be a lot more than the stated fee!
Look for a ticket with multiple destinations and you can fly into one airport and out of another for about the same price as a round trip ticket to Madrid. One popular option is to fly into Pamplona and return from Santiago. American Airlines has code sharing with Iberia Airlines, and serves those airports.
Buen camino.