How much to tip in order to get a better room in Punta Cana, Dominican?

My gf and I are going to Punta Cana in March. We have booked the vacation with a travel agency that has a deal going on right now... There are 4 separate resorts that are all 5 stars, but we cant pick a certain one.. nor a certain hotel room. The company picks for us. This is part of the deal, in exchange, the price was reduced which is a better thing in my case. Now I've heard that if you don't like the room your staying in, then just head down to the reception desk, be nice and friendly, then tip them and ask for a better room if possible. Depending on the tip, the room varies. I would like to know how much is a good tip for a good room.. just a rough estimate. or if any ones done this before. thanks PS. I know that this resort takes tips... because it says it on their website.