Putting a dollar amount on it is almost impossible. Each casino has many different ways of deciding it. I did find an interesting article that lists some steps to help you get a comped room.

Step1
Write ahead to see if you can set up an account for yourself at the casino. If you have about $5000 to $10000 to send ahead or put in the account when you arrive, you should be able to get a free room. Most people wonder why someone with an additional $5000 to $10,000 would need a free room. They don't need it, but saving money is the way they accumulated it. It is also part of the game of maximizing money.

Step2
Draw a few thousand out of your casino account on a daily basis to make it seem like you are loosing. Make certain that you have played a bit at each table. Win or lose, remove the money and transfer it to the safe in the room. Find a place in town to get money orders and, if you choose, convert it to a money order to yourself. It is more comfortable and safer, especially from any urge you may have to spend it.

Step3
Check with the casino host periodically to see if your room is comped.
Drawing money out daily doesn't always work because casinos have found various other methods of tracking losses. Asking for the comp and what it takes to get the room comped may be all you need to do.

Step4
Get a slot card to a local casino that's affiliated with the one that you're staying at. Many times they have programs that will add dollars for use when you go to a sister destination. If you've played on the card at all, then go to the website and book a room. It will show you if your room is paid for by the casino. This is usually one of the best ways to get a room at a casino for which you hold a players card. If not free, there's usually a discount.

Step5
Use the players card every time you play. Points to get a room comped are usually accumulated that way. If you're at the table, make certain that you bet higher when the pit boss is watching, unless you're on a losing streak. Comps on tables are based on average dollar played and length of time played. If you're betting $50 each time the pit boss looks but go back to the table minimum when he isn't looking, your average play will be recorded as $50. Cameras also pick up the action, so you have to be discreet.

Step6
Get several slot cards and have everyone in your party use one of the cards. Building up slot points on one card is better than on many. It allows the comp to grow enough to get a room. Most people that get comped because of play don't refer to it as a free room, but as a "prepaid" room, due to losses.