I would suggest you take basic domain registration charges pending for last 3 years along with say 10-20% extra for renewing the domain. Say 10X3 = 30 + processing charges of say around 5-10$ so in all I would not charge more than $40.
The domain in itself cannot be counted for valuation purposes and I would resist myself from asking a price on the domain because the client may simply change the name to something else or even wait for it to drop. If something like that happens you will lose out all the money you paid till now.
Time and again I have made posts at various locations that web developers should not get in to this business of registering domain and hosting for a client. Best is have a reseller account with some leading company and simply direct the client to that site - if the client registers via you fair enough but in the end you dont end up holding customers domains and then waiting for the client to pay up.