Dear Sharad,
First fix the eligibility criteria ie., employees who have worked for 1 / 2 / 3 yrs. As far as the price is concerned sit with the finance / accounts guys, find out the book value & accordingly decide or organize a meet, go for auction, keep all the computers in an open area, give identification codes to each PC, allow the employees to check the systems and ask them to submit their bid, form a committee ( ensure finance head / Manager is part of the team) and allot to the highest bidders & close.
Regards
Kameswararao