I buy surfactant for my herbicides. It’s cheap. However, a surfactant won’t do you too much good for spraying your fruit trees. Surfactant (i.e. stands for surface acting agent) are just wetting agents (i.e. spreaders) in the context we are discussing them. A wetting agent can help a little in the distribution of insecticides/fungicides on the leaf tissue, but the benefit isn’t all that great. They are good for herbicides because you want the herbicide to contact as much of the foliage of the weed as possible for maximum absorption.
Spreader/stickers (like biofilm, Nufilm, ect) have a wetting agent along with a sticker. They are mostly used during the growing season to try to protect fungicides/insecticides from being washed off during rain.