I work as a retirement actuary. It is a complex decision that depends on a bunch of information, some of which you have provided, but additionally you'd need to consider the following:
Plan Information:
Any early retirement reductions applicable in the plan (e.g. is the 55/20 pension reduced at all? What about the any and 23 pension?)
What pre-retirement death benefits does the plan have, if any?
Does the plan guarantee a return of participant contributions to the plan? If so, would the 70k count?
What payment options does the plan offer (e.g. lump sum, life annuity, joint and survivor annuity, life annuity with n years guarantee)? What option would you most likely take and what if any reductions apply (e.g. typically joint and survivor annuities are reduced somewhat to provide additional coverage to your spouse)?
What happens to the 70k if you quit, die or become disabled prior to retirement)?
Are you fully vested in the pension plan? (I believe answer is should be yes)
Non-Plan Information:
Your current salary
Your projected salary at retirement
Are you in reasonably good health and expect have a long retirement after you retire, whenever that may be?
What rate of return do you expect to earn on the 70k if you do not buy time in the pension plan?
Any spousal/family information that may be relevant to your decisions regarding the pension.
As another poster mentioned, can you pay the 70k pre-tax?
There are probably even more considerations I'm forgetting.