Keep in mind that "to be tantamount" is about being equivalent in significance or effect, not about being equal in value or amount.
Please take a look at the following examples:
- "$3 is tantamount to half of his weekly allowance" (x): you are comparing the amount, not the significance of $3 to this person. So "tantamount" here sounds awkward. You should say "$3 is half of his weekly allowance" instead.
- "Spending $300 on this video game is tantamount to wasting all of my monthly budget on something useless" (O)
