happykell, have you checked out the menus online?
allears.net has them, as does dlresortinfo dotcom.
Many people find it easy to share an adult sized meal, and many find a kid's meal (such as the bean/cheese burrito meal at Rancho del Zocalo, or the penne pasta meal at Plaza Inn) more than enough for an adult. Both of those things reduce the cost, if they satisfy the people in your group.
You can bring water, or even just get cups of ice water from the restaurants, even if you don't buy anything else. There are even places to re-fill a water bottle (one I can think of is inside the Rancho del Zocalo seating area, there's a faucet/fountain thing on the "cart" that has the salsa and napkins).
For our family (hubby and I aren't thin by any means, but we also aren't big eaters) we can share meals, I can get buy with a kid's meal easily, and we have a 3 year old who varies...we're also vegetarian so things like prime rib aren't on our menus or pulling on our pocketbooks...I budgeted $100 a day for everything on our last trip and we didn't even hit that mark.