To output a string array such as:

[
  "1: 34.45",
  "2: 21.67"
]

try: Account.Order.Product#$i.[$join([$string($i+1),': ', $string(Price)])]

To output an object array such as:

[
  {
    "1": 34.45
  },
  {
    "2": 21.67
  }
]

try: Account.Order.Product#$i.{ $string($i + 1): Price }

JSONata playground: https://jsonatastudio.com/playground/9b835d23