Melissa, this is very kind and nice of you. He will be paid and paid nicely. You are paying the agreed contracted labor price and a good job should be done anyway.
But, if this person is going out of his way and above and beyond the contracted price, then verbally praise his work and let him know how happy you are with the work. This does go along way, belive me, I almost love this as much as the $$$.
But in answer to your question, $50 would be nice and very appreciated. I would not go any further though.