I recently ran a similar analysis. I won't go into the details but what I found was that, applied to your case, if you retire or leave the public service before age 59 (after 35 years service) then the buyback is worth the investment. If you retire at 59 or later then the buyback doesn't pay off. It's basically impossible to plan that's far in advance in my opinion so the decision is a bit of a crap shoot.

It's also worth considering that when you're starting out in life (in your early 20s) any extra bit of cash can be useful for buying a house, traveling, paying for a wedding, buying a car etc. depending on your situation. If you work a full career in the public service, your pension alone will allow you to have a comfortable retirement regardless of whether or not you do the buyback now. Just my two cents.