I would think you would use a discounted cash flow model. However, there are a couple assumptions that will change the answer by a lot.
1. How long do you plan on getting this pension? Say 25 years
2. Is there a cola? for simplicity I will say no
3. What interest rate? This is a key input, and one of the hardest IMHO to determine. A safe rate, say the rate of US Bonds if you feel the pension is absolutely safe, or maybe a higher rate if there is some risk of not collecting it.
So a 25 year pension at 4% at $35,000 a year comes out to $546,772. If you are going to use a cola'd pension, then you have to escalate the pension by your forecast inflation and discount each value back to present value.
In my example there was no residual value to the pension. If there were you would add that to the 25th year of income.