Just understand then that you're paying for the experience, and get the cheapest cards you can get to crossfire. Once you get it up and working, then put in your actual GPU you use for gaming.
the reason that I'm saying this is that crossfire(and SLI) never produced exactly what was promised. The drivers are buggy when using multiple GPU's and also not all games support multi-gpu's. That's why I say to get the best single card you can afford. If you decide 3 years for now you need an upgrade, then SLI or Crossfire may be a viable option because you could probably find a used GPU to match yours and get a reasonable upgrade for very little money compared to newer GPU.