Hi Susan! Welcome to the Disney Parks Moms Panel. Your question is a slight bit difficult, as finding a restaurant that is "not too busy" at Magic Kingdom can be tricky, and cost is very subjective. I've taken a look at the menus on the Disney Parks website for all of the Table Service restaurants, and
The Plaza Restaurant seems to have the lowest prices for entrees. They start at $10.99 and go up from there. I strongly suggest you make an advance reservation, and then you won't have to wait too long for a table. If you decide not to make an advance reservation, try to dine at an in-between hour - when it first opens, or towards the end of the lunch serving time or very beginning of dinner service. Also, The Plaza Restaurant may not be at the top of most people's lists for Table Service Restaurants at Magic Kingdom, so you may encounter lesser waits - but I can assure you that I have heard very good reviews of the food served there, so no worries.
I hope this information helps with your planning, and please don't hesitate to ask me any other questions that come up while planning your Walt Disney World vacation. Have a magical day!
Kristen