Unfortunately the dayrooms are way over-budget for the OP's specified maximum 1500 THB even for the minimum time-block of 4 hours. And if 8 hours or more the amount they charge gets to be silly. If you want a room for that long, it makes no sense to not get a room outside the airport, unless entering Thailand is a hassle or problem for you. For 4 hours, I suppose it's debatable as to whether one is better off with a dayroom or going outside. I'd probably opt for the dayroom myself being you'll lose considerable time going in and out of immigration and in a taxi to/from the hotel.

If traveling alone, I'd personally take the train from Suvarnabhumi to Makkasan or Phaya Thai, and then a taxi from there to whatever hotel I chose. For Makkasan, a round trip ticket on the express train was only 150 THB I believe when I was there two weeks ago. For Phaya Thai not sure what the fare was, but for me even if it's the same price as a taxi, I prefer the express train. No worrys about my safety with the way some of the taxi drivers drive on the expressway. If traveling with two or more people, the cost of the express train multiplies per passenger, whereas with a taxi it's the same, so if not traveling alone the train can actually be more expensive than a taxi. During non-peak travel times, the train probably won't actually save you any time, but I still prefer it. During rush hour it can definitely save you considerable time if your hotel is located conveniently to one of the stations. I do not recommend taking the commuter train to/from the airport if you have a lot of luggage. It can be quite crowded on that train.