Make sure you know the measurement of the biggest painted number, even if it means going around to all 500 units.
Then price aluminum big enough to cover the numbers.
My supplier will cut a 4'x8' into whatever size I need, even an odd size, for a nominal fee.
You could probably get away with .040
That way you can get more blanks for less money than if you were going with a standard 12"x18" blank.
You might even want to bump up to polymetal for a small bit of dimension.
I'd letter with cast vinyl, probably black, for longer lasting signs.
Figure your materials and go from there, including installation if you are the one doing the installing. (can't help you with that part) Remember to bid enough so you make a profit, instead of just covering your materials.
They are probably collecting several bids, and if you detail why your bid will be better, so they can compare apples to apples, maybe you'll land the job.
Love....Jill