That's cheap! The launch'll set you back about $100m so another $3.5m for propellant isn't going to be worth worrying about. At some point we'll be wanting to start finding more fuel for our rockets at our destinations. There are ways we know how to do this for oxygen, hydrogen, and methane but getting more xenon on Mars or Titan just isn't doable. So at that point I expect people to switch over to Argon which doesn't ionize quite as easily but can be found all over the solar system.