It's not cost, in my opinion, it's an insurance move. Gatwick cannot afford to have an incident that damages a plane; that leads to serious questions of competence surrounding why the airfield was kept open when it was dangerous to do so, and though I don't have the regulation in front of me I'd wager they are actually obliged to keep the airfield closed in this kind of circumstance.
Yes, Gatwick is a business, but this wasn't a straight revenue-loss decision, otherwise they probably would have taken the hit and run planes anyway.