Ask them how much they spend on paperclips, pens, coffee for the coffee machine.
Try to see what they have spent money on and justify why your domain has a greater benefit.
Unless the only reason they want the domain is for SEO purposes then it's pretty much pointless to ask for more I think? Now if you are confident they will use your domain as their main website address then as for 1% their marketcap.
Since you said "marketcap" my suspicion is that it's a crypto domain?
Am I right? Either way good luck!