We've found it to be a lot less than that generally - almost half your estimates.
[Edit] re-reading, I think your flight costs are accurate from Chicago to Spain. They are less for us because half the team travelled from London. If you are all in Chicago, you should go to Canada or Mexico or Central America - lower cost to achieve the same goal.
You arguably don't have to go that far from home I guess - depending on where you live - just needs to be "away" and a great place (and have great Internet!)
[Edit] re-reading, I think your flight costs are accurate from Chicago to Spain. They are less for us because half the team travelled from London. If you are all in Chicago, you should go to Canada or Mexico or Central America - lower cost to achieve the same goal.