Well, there is no rule. Choose the price you are comfortable with paying for this domain and make an offer. You never know who is on the other side of the deal: he might be happy to get rid of the name for $100 or even less, or simply he checks the demand (also he might just forget to set up a minimum offer).
Useful would be to check the price for a domain name across main platforms (Sedo, Afternic, Dynadot), before making an offer.