Tickets for the June 27 concert went for as little as 18 euros ($23) -- about half the normal price.
The American rocker told El Mundo they originally planned to skip Spain as the economic crisis made it unlikely the show would sell if fans couldn't afford tickets.
"I didn't want to let fans down," he said. "We did a study and we saw that due to the economic situation Spain wasn't going to be on the roadmap. But I didn't want to leave out fans from a country I love that has treated me well for 30 years."
In other words, the band is performing for free, and the venue is only charging enough to cover costs of staging the show.
The tickets did sell out.
Spaniards are suffering myriad economic woes, including a deep recession marked by staggering unemployment of nearly 27 percent.