There is no hard and fast rule regarding when it is commercially beneficial for you to transfer your payments from the PAYG (Pay As You Go) option to the Perpetual option, which has a one-off license payment and then fixed, monthly payments for on-going support services & future software maintenance.

The best calculation model could be to use the Equivalent Annual Cost method, which uses a Discount Factor to calculate the Discounted Investment value.
Calculate EAC

Using the following fees, cost of capital, a nominal 'lifetime' and monthly fees of $250 

Investment cost of $4,500.00
Expected Lifetime of 3 periods (years)
Annual Maintenance cost = $3,000.00 ($250/month)
Cost of Capital = 5%

The Equivalent Annual Cost would be:

EAC  = Discounted Investment + Maintenance Cost
        = $1,653.07 + $3,000.00
        = $4,653.07

Now use the EAC to compare with your annual PAYG charges (remember to allow for GST).

If you are paying more PAYG than the EAC value, the algorithm would indicate that it is a better option to purchase a Perpetual license.


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 =   $4040
    EAC at 10 years = $3582