The value of your donation is based on the fair market value
of each item you donate. Fair market value (FMV) is the price that your items
would sell for on the open market such as at a garage sale.
You can try Its Deductible for a list of
laptop values for Mac or PC.
https://turbotax.intuit.com/personal-taxes/itsdeductible/
Alternatively, you could look at ebay, craigslist and other
used computer equipment websites for an approximation of the laptop value.