Ka-ching is an onomatopoeic word used to describe the sound of a cash register opening and closing. However, if you're looking for synonyms to substitute for the word "ka-ching", there are a few options to convey the sound of money, such as "cha-ching," "clink," "jingle," and "ding." Additionally, you could use words that describe the action of making money, such as "cash," "profit," "earnings," or "revenue." Other options include slang terms like "moolah," "dough," or "greenbacks." Overall, there are a variety of synonyms you can use instead of "ka-ching" to express the joy of making money.