I will take LZ's case as example, and put my understanding as following. Please correct if I am wrong......
First, your pension entitlement will only start once you have completed 2 years working for the GoC fulltime. Otherwise the contribution+GoC match portion will be paid out to you upon your departure.
2nd, say you have worked 5 years, you have contributed $100X26x5=13,000 (numbers are just for illustration purposes here.)
And suppose your highest 5 year average salary is 60,000 (this number will be higher the longer you work for the Fed). So your pension formula will be:
60,000x2%x5=$6000
This is the ANNUAL amount you can collect once you reach the age of 65 or have worked for the fed for 35 years. So say you live until 90, and you leave the Fed after 5 years, and keep making your nice dough from private sector. When you retire at 65, in addition to your private sector nestegg, you can also collect $6000 every year for 25 years only because you worked for 5 years for the government. This 6k will also be adjusted by CPI annually. So you would receive:
$6000x25=$150,000
v.s.
your contribution of $13,000
Well, you do the math
Another point, check to see if your previous workplace has Pension Transfer agreement with public Service pension plan, if yes, you are able to transfer over the previous pension and add more years to your service.