SUBSIDISED TRAVEL
Give club members free travel, or specially chartered flights and accommodation.
They could even pay only 20% of flight/accommodation costs, and have free tickets for the games themselves.
Spanish football tickets often aren't expensive, vis a vis English Premier League games. However, to travel to the USA, arrange a visa, and accommodation can be costly.
Accordingly, this move has gained some ire in some quarters, due to the aforementioned factors. Fans also have families, jobs, and lives beyond supporting their teams. So going to the US, even for a few days even, could cause unnecessary disruption.
Nonetheless, Girona has stated that it may consider its game vs. Barca in Jan 2019 as an early exhibition or example. Girona, like Barca a Catalonian club, is far smaller than Barca (or Real Madrid for that matter) and would welcome the opportunity to grow the club and the brand.
There may be other smaller clubs that would turn around, or at least be warmer to the opportunity. They arguably need the monies more than Real Madrid and Barca. At the time of writing though, Real Madrid is against the notion.