I'm often amazed at the extreme differences in the cost of beds, especially when going from a queen to a king. That's why I think that making your own bed can be extremely rewarding.
The number one factor in making your own bed is sizing the project to allow for your mattress. Listed below are standard mattress sizes:
Twin: 39" x 75"
College Dorm Twin (Twin Extra-Long): 39" x 80"
Double: 54" x 75"
Queen: 60" x 80"
King: 76" x 80"
California King: 72" x 84"
Use these dimensions when planning the interior space of the bed frame. I like to add 2 inches to each dimension, which will give 1 inch around all sides, to allow for bedding. Notice this example for a twin mattress below.
For platform beds, I like to allow a bit more space than that.
Height can vary dramatically and is up to your preference. I've designed platform beds as low as 10 inches. I like to aim for a height that will allow the mattress to rest at 16 to 26 inches above the floor.
Here's a platform bed project I created for DIY Network - Modern-Style Platform Bed. Notice that it has a center support leg underneath.