I pay $5 for complete desktop computers with hard drive or not. If any other parts are missing price goes down. As far as the hard drive i would ask each seller what method they prefer as there are several options. I wouldn't mention these options if they don ask about the hard drive.
1. you keep the hard drive along with the computer, best option
2. you pull the hard drives at time of drop off and hand them to the customer, good option
3. you disassemble the hard drives and destroy the platters, getting worse
4. you pull the har drive and pay a company with certification to professionally destroy them, worst option I would charge a fee or deduction from price for this