Depends on the area. I have two stumps that are probably much bigger than yours. Been waiting for a guy I know to do it;
he was going to charge me $100 for both. I told him I'd give him $100 each. Another guy, who already had the equipment at my neighbor's house (so I figured he might give me a good rate, since there was no extra travel, etc., just do the job as a quick incidental job) gave no discount at all; wanted $200 each.
For $500, you could rent a small backhoe and go to town on digging them out.
http://www.bigrentz.com/equipment-rentals/backhoes/?gclid=CLGz2Yzfgb4CFcMDOgodQiUAng Or, you could rent the stump grinding equipment yourself; though it takes a little more skill than people might suspect.