Day passes are only usually more expensive than a room with MORE than 4 people going. On the days the day pass is higher priced, the rooms are also higher priced.
As an example, I can take my kids on a day pass pretty much any weekday this month for $55. There are 3 of us right now, so $165. Day pass gets you in open to close, so 9am to 9pm most days. You cannot access the water park until 1pm on check in day. They are VERY strict about that at the Anaheim location.
With a day pass, we can eat breakfast and dinner at home, just needing to buy lunch there. We don't have the hassle of packing. Just put swimsuits on under our clothes, grab the swim bag and go! It's way more appealing than dealing with all the logistical aspects of an overnight stay with kids. We can be there in 15 minutes. Day passes are the best thing to happen to GWL, in my opinion.
One room on a random weeknight is list price $139.99 (using a 30% discount code). With taxes and fees added in, it comes to $222.36. Then, since we will be staying overnight, you have to add in more money for additional meals. Staying one night runs us close to $350-400 after you factor in food, parking, the inevitable arcade and bowling, ice cream, breakfast the next morning at Dunkin Donuts, etc.