How much should I offer on a vacant lot?

I have my eye on a 0.55 acre lot listed at $59K in rural south Texas (Jackson County). The sellers are selling this 0.55 ac out of their current 0.84 lot with their home on it. I feel like they are asking way too much for the 0.55 ac but I also do not want to make them mad if they think I am lowballing by giving them a realistic offer (12-15K?). How much should I offer for the 0.55 acres if their homesite land value (0.84) is only $8,820? Is there a formula for making offers on property based on tax value?