That's a good question. If that chip in similar condition has sold on ebay in the last 3 months, market rate is easy. If it hasn't, but I've seen a similar condition chip sell in the past 12 months or so somewhere (ebay, forum, chip chat auction), market rate is pretty easy there also.
If the chip has not sold anywhere public in the past year, I'm not connected enough with people and private sales to have many people to rely on for those. Most chips I want aren't the "private sale only" type anyway.
If none of that info is available, hopefully I'm working with a candid seller who can say "I got this chip for $12 eight years ago and these kinds of chips have gone up/down/ about 50% since then or something like that.
Absent all those, we make something up!