I have a 28 x 18 garage/ shop which is basically just studs... Im wondering what would be the easiest and cheapest way to insulate it so I could do some work in the shop this winter... I would be using electric style heat due to insurance purposes... I was thinking plywood walls and ceiling... Heres a few pics, let me know what you think...

http://www.servimg.com/image_preview.php?i=2&u=13136234

http://www.servimg.com/image_preview.php?i=5&u=13136234

Cheapest insulation would be batt insulation. I'd go with drywall before plywood, easier to work with and cheaper. If your concerned about banging up the drywall use plywood on the bottom 4 feet of the walls.

So, should I build a flat roof and insulate or insulate up to the peak... and have no flat roof?

Do you mean build a ceiling or not? If so, it would help keep the heat from rising up in the rafter space.

Yes, that's what I meant... "build a ceiling" I would have to add some more ceiling joists, I think the few that are there now are about 8 feet apart from each other... And I'm not even sure about walking on the ones up there already...