I came up with a good solution for making an inexpensive but effective cabbage cage. Look at your local discount store, dollar store or department store for collapsable mesh laundry baskets like these ones:
Get a pair of scissors and a good pair of wire cutters. Remove the top 1/3 or so of basket. Turn it upside down and stick it in the ground over your plants. Voila, it keeps the white butterflies away and it doubles as a perfect sun shade.
Some plants will outgrow it, and the mesh is probably too big to keep out all bugs, but for 2 or 3 bucks, can't go wrong here.