Honestly, if you can get in you really don't need anything at all.

If you happen to have ~$50 to spend on a "Pirate's Frigate (Lv 1)" this will grant you an immediate boost to you HP and card options in Gauntlet battles.

All and all, I'm not seeing very much here you CAN spend money on beyond ships. "Founding pirates" sell for like $3000 and, as many quests say they need up to 20 pirates, I think in time pirates will be unlockable fairly easily?