How much is the cost to fix a rump-sprung chair? [View all]

I have a mid-century modern chair that I adore. The coil in the seat (I don't know if that is what it's called) is sprung. Anyone know what the likely cost is to fix such a chair?

I don't want it re-upholstered. It's original -- the fashionable green of the era.

Thanks in advance for any advice.