K10Ds used or refurbished prices range from $350-$400.
A new K20D you can get for $650-$700...but for the $250-$300 difference:
1. You get a new up-to-date camera which will still resell good when new Kx0D arrives.
2. You get a bigger 2.7 LCD
3. Dust alert feature/ dust mapping
4. Fine-tuning of AF on lenses
5. Larger megapix
6. Debugged K10D "base program"
7. CMOS sensor which gives more accurate color repros
8. Higher ISO
9. Live view (even if half-baked)
..and others I failed to mention.
If you can justify these upgrades as $250-$300 worth, then I guess you know what to do.
For this price of an upgrade,it's like buying a better lens but..you get this all in one camera body (which will make a huge diff in all your lenses) instead of just one lens upgrade.