The best calculation model could be to use the Equivalent Annual Cost method.
Using the following fees, cost of capital, a nominal 'lifetime' and monthly fees of $200
The Equivalent Annual Cost would be:
Use the EAC to compare with your PAYG charges (remember to allow for GST).
The 'Expected Lifetime' value is a bit of an anomaly as there is no future capital reinvestment required for the Perpetual license.
If you extend the Expected Lifetime out to 5 or 10 years the following EAC values are calculated:
EAC at 5 years = $3439
EAC at 10 years = $2982