I bought Palm Pre in April. While still good I decided to switch to Android and bought Droid 1 off ebay. Sold my Palm and basically it ended up being $60 that I had to pay to upgrade. When I bought Palm it was $50 and Droid was $200. $150 difference then, it's only $60 now. Not too bad.
Obviously it may not work out that way each time, but for example if you buy D2 or Incredible (~$300 on ebay) and sell D1 (~$130 on ebay), the difference will be about $180 or less than you'd pay for D2 if you were eligible for an upgrade. Add about $100 if you want Dx.