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. Starter is reported to include 999 Cash, so check the Codes page before a longer early grind.
The exact cash payout formula, machine rates and shop prices are not available from the current public evidence. This page provides a testable routine, not a guaranteed cash-per-minute number.