Rather than watching for sales on dishwasher detergent, you can mix your own in a flash from ingredients you might already have on hand.
2 cups baking soda
2 cups Borax
1/8 cup OxiClean
The laundry size Arm and Hammer baking soda and Borax boxes together cost less than $7 at Walmart and will make multiple batches of dishwasher detergent (and laundry detergent, too). A store brand of powder dishwasher detergent would likely cost at least $7 for two boxes that give you far fewer dishwasher runs than the big boxes of Borax and baking soda.
I usually have a box of OxiClean on hand. I haven't had good experiences with other brands of oxygen laundry cleaners, so I stick with real OxiClean these days.
Store the dishwasher detergent mixture in an airtight container and dispense exactly the same as you would with regular detergent, into your dishwasher's detergent holder.
If you like to use a rinse aid, try using plain, white vinegar in place of a commercial product.