The cost of ownership is small compared to a a Pc laptop. Buy each upgrade and sell just before release of new model average return will be about 60% of original cost which means you iPad will cost you less than$1 dollar per day average if you are smart.
For some strange reason, when I was looking to sell my original iPad quickly online (Gazelle.com), I watched the pricing rise for my model until one week before the new iPad started shipping. That's when the price actually peaked. I sold mine at that time and watched the prices fall again after the launch.
I agree that if your iPad is in great condition (no engraving and no scratches on the back) just sell it every year and get the new model. It's indeed very cheap rent.