Two used G4-based Mac Minis: $550.
One 17″ monitor: $180
Cheap keyboard and mouse: $30
Two resistors to enable the G4s to run headless: $1.90 (for two packs, one at 270ohms, one at 120ohms).

Instant testbed environment to permit simultaneous debugging MacOS X software for v10.3, v10.4 and v10.5 compatibility: Priceless.