Among other alterations, the €4-billion project plans to move Stuttgart's main railway station underground and turn it round by 90 degrees. It also requires the construction of 57 new kilometres of railway track.
Opponents are likely to point out the potential savings as well as the potential funds for alternative infrastructure projects should the construction be stopped.
Much will depend on chief mediator Heiner Geißler, who is due to deliver a verdict on Tuesday.
During Friday's negotiations, Baden-Württemberg state premier Stefan Mappus heavily criticized the demonstrators, and particularly the state Green party, accusing them of giving the impression that money not used on Stuttgart 21 could be spent elsewhere.
Mappus argued that since there were no concrete plans for other infrastructure projects in the region, the federal budget for Stuttgart 21 would simply be used in other parts of the country.
Mappus said that Baden-Württemberg's transport sector was chronically under-financed, and that the state needed to take this opportunity to improve the local transport system.
The Green party's opposition to Stuttgart 21 is giving it a boost in the opinion polls, and it is currently the second strongest party in Baden-Württemberg after Mappus's Christian Democratic Union (CDU). The state will hold elections next spring.
The government was forced to hold the mediation talks after more than 100 people were injured during a police operation against the ongoing protests. Over 40 protest groups are again expected to converge on the Mittlere Schlossgarten in central Stuttgart on Saturday.