That is to replace the cost of launching one SLS. If you include development costs cancelling out SLS now would easily save $4B per launch + 10B+ of dev costs.
I think the proposal would be to open up funding for both additional launches AND additional science missions. For the let's say $6B/year or so, you could do a huge amount.
The only reason SLS is required is because the mission architects HAD TO use SLS in their architecture.
SLS in terms of what it does for it price would never be used in any architecture that tried to achieve the most with a given budget.
F9 Heavy can do a lot of good science missions, and many payload sizes are coming down. I'd personally include the Ariane stuff as well, James Webb launched on a Ariane 5 (around $200M per launch - so can get 20 launches on that or 10 launches + 10 new $200M science missions for one SLS).
This ignores the DEVELOPMENT cost of SLS which is mind bogglingly large as well as some of the ongoing just sustainment costs (also insane).
For example, take 4B/year from SLS. You could fund just an incredible amount of activity with that.