If £120,000 joint salary is is correct, bearing in mind you have received what would have been your contributions in your pay, your actual loss is therefore £150,000 (same caveats around the calculation of this figure as before) minus the extra money in your pay packet each month.
Bear in mind pension contributions are before tax (so you don’t get the whole amount if you don’t pay into a pension - the government takes their cut)
If you are both on 60,000 a year, the civil service contribution is 7.35% (5.45% between £34,200 and £56,000).
That means you have opted out of paying £4,410 each per year. However you will have received the money after tax. That means less 40% tax, you will each have received an extra £2646 a year in your pay (I’m ignoring national insurance).
That totals £21,168 over 4 years for both of you.
Therefore what you have done is taken an extra £21,000 now in exchange for an extra £150,000 in the future for a total loss of roughly £130,000.