So you must be comparing the pension increase to keeping the 70K in IRA, are you using a realistic factor for the IRA 70K amount regarding growth, and how about the inflation factor, since I think the Pension is Cola'd?
Let me see if I can explain... To make comparisons easier, I am using a fixed 6% rate of return on my IRA. Obviously, market fluctuations will be different.
For the baseline run, I simply keep contributing 6500/year to my IRA, and use her calculated pension amount (her retirement system has an online calculator to get easy and accurate estimates).
For the extra service credit run, I start with the same current 73K balance, but stop making contributions. Those contributions would be redirected to a new IRA for my wife. My IRA would grow slower, and hers would be used strictly to roll over into buying pension service credits when she retires.
So, her IRA is essentially out of the equation, other than the four years of extra service credits it would buy at retirement. I took the baseline pension payment, added the increase from the additional service credits, to compute the new higher pension payment.
Then I simply plugged in the new higher pension payment, and removed my IRA contributions from now till retirement. Social security payments stayed the same.
The end results were fairly close, but buying the additional service credits didn't make a huge difference until much later in life (probably after I die). The results changed slightly with different rates of return on my IRA, but not enough to be significant (less than 1 years income).
When do you have to make this decision ?
We can't purchase the extra credits until she retires, about 7 years from now. However, we would have to start a new IRA and redirect our contributions to her IRA now if we expected to roll it over into the service credits.