There are a number of ways to repair holes in plaster walls. Some are better than others. I find this one, if done carefully, is invisible after being painted. Total cost of material is less than $25 for the first hole (1200x900 plasterboard repair sheet, liquid nails or aquadhere, polyfilla or spakfilla). The next six or seven cost nothing as you have all the stuff you need.
Thanks for sharing @Oldfart.
The following video might also be helpful for Workshop members who need to repair a hole in a plaster wall.
Great suggestion @BigSteve, I also recommend using Cornice Adhesive, instead of Spakfilla, Polyfilla etc, as it doesn't slump as much if filling a wider vertical gap.
Is this also the best way to deal with a large crack in the wall? Or should I just fill it?