I would think that is somewhat excessive given the little site...

I would think that is somewhat excessive given the little site disturbance the Well Fields creates and that would be cleaned up as they go on; and the modular nature of the Ammonia - Urea Plant. Guessing in 30 years time $100M with inflation could be it. By 2050 converting it to solar produced Hydrogen - Ammonia - Urea could be the thing so no need to remove the Plant,

For a long time the Crown covered the rehabilitation risk of mining in SA hence the prevalence of a lot of old sites like Nairne Pyrities at Brukunga, Uranium Coarse tailings at Radium Hill and countless old mines like at Moonta, Kadina, and throughout the Flinders Ranges.

If you read this and take a good look at what's been done (Google earth) then it's easy to speculate what the Crown may still have to deal with for years to come after all it was mostly their doing and who would not be surprised if true due diligence was done by all the parties.

Leigh Creek mine remediation | Energy & Mining (energymining.sa.gov.au)