Very interesting. It's probably because fewer people take the time to counterfeit $50s, $10s or $2s than anything else. What about $100 bills? In Argentina, if you have an older $100 bill, no one will take it. And apparently there's a roaring trade in fake $20s in Costa Rica, which I only learned at a casino there recently when I took USD directly out of an ATM and had it inspected by a pit boss in the same establishment. It's ironic, because if I were someone with an interest in counterfeiting, I'd focus on forging Pesos or Colones or something no one looks at before I'd take a stab at USD.