Will you do 'the right thing' i sthe question?
I'm going to go out on a limb and suggest this isn't a simple once-off human error. They advertised $1800 on the website for months (it's still up there). They cut checks for $1800. They took photographs of winners holding checks for $1800. Then they handed the winners the checks for $1800. This isn't like the items that sold on Amazon for a penny due to pricing software errors, or a typo on the check. The "bad" ($1800) number was advertised and reviewed by multiple people associated with the race on multiple occasions. I don't think this is OP's problem, so it's down to his discretion whether or not he wants to help the organizers. Given the incompetence and downright nastiness in their conduct, I'd decline the opportunity to help them, especially via direct cash payments!
And now they decide oops they REALLY meant $900 when they wrote $1800.
I wonder if they would have been so "aggressive" if they erred in the other direction ? I've won a bunch of expired coupons or other "prizes" that are impossible or near impossible to claim.