Are you sure you understand this correctly? Per diem usually means that hotel costs are reimbursed up to the limit, not that you are given the money regardless of what your actual hotel cost is and without submitting hotel receipts. [Similarly, one is typically reimbursed for actual airfare and rental car costs (or the employer purchases and provides the plane ticket directly through the corporate travel agency.]

Don't forget that TPS earns at half of the usual rate for Bonvoy points (like RI etc.). IIRC you don't earn points on any miscellaneous spending shared to your room at TPS either.