How much would you say a decent box truck should cost in this market?

Age is just a number. Look at the miles, that's a better indicator of the wear and tear. Your suspension parts can sit for a decade without any appreciable deterioration. But 200,000 miles in one year and they may need attention. Depending on the motor, under 250,000 miles is preferable. Much more than that and it's getting close to wanting a rebuild. A decent price would be $40-50,000. Don't limit yourself geographically. Just like cars, the same make and model may have a large variance in price depending on location. Be willing to travel across the country to save a few thousand.