I use onion almost every day in one thing or another.
In the fall and winter I try to buy small boiling onions at the farmers market, two pounds for a dollar. The little onions are just right when cooking for one.
When I do have to use part of a large onion I keep the remaining onion in a glass jar with a tight fitting lid, in the fridge they last for several days.
I also prefer to use powdered onion in some recipes. I find that when I make cabbage salad the powdered onion tastes better than the fresh.
The best money saving tip I can offer is to use your imagination and every morsel of food you buy, eliminate waste! If you still find yourself tossing food then cut back on your purchases. IMO it is not a sin to run out of something, but it is a sin to waste it. That concludes my sermon for today!