The practical route
Choose exercises you can complete reliably, watch the cash change shown by the game, and reinvest only when the current purchase clearly improves the next training loop. This Ultimate Gym Game cash spending guide uses the public Trello claim that exercises give one dollar per rep and cleaning around the gym gives money for store purchases.
Starter is reported to include 999 Cash, so check the Codes page before a longer early grind. This page still avoids a guaranteed cash-per-minute number because speed, setup and updates affect the result.