Good find. If you want a nearly universal technique, snatch up a hex code viewer and get multiple save files with the fewest changes you can manage between them, with the variable you want to switch being one of those changes. (So, for example, in this game you might want to run the same day exactly the same way except for buying one watermelon in Save A and five in Save B.) Then compare differences in the save files, keeping an eye out for the specific numerical quantity you were targetting. (IE What your cash was when you saved in both instances.)
This works across a wide, wide array of save file types - jsons, homebrews, rawtext, *.sav, etc etc etc