When determing the retail value of a used tub, I always use these rules:
First Year Depreciation 20-30%
Each Additonal Year- Additional 10% depreciation
For example, if he paid $5000 for it 10 years ago, today it would be worth:
5000 x .75(25% depreciation)=3750
3750 x .9x.9x.9x.9x.9x.9x.9x.9x.9=$1452.00
Retail value of $1452.00 if he paid $5000 for it 10 years ago. Hope this helps.