"would we save $BIGBUCKS by building on what we have instead of starting nearly from scratch?".
In short - often no.
Nobody sane thinks that you can launch SLS for under 2 billion dollars per launch.
This is a launch cost of $30000 per kg of payload.
Falcon 9 can launch the same payload (admittedly split into several) for $5000/kg.
Falcon heavy (debut flight expected within several months) launches can currently be bought for around $1500/kg.
SLS 'benefited' from congress - who at best have a passing knowledge of rocketry, but a very good knowledge of who makes existing hardware in their constituencies mandating that it use shuttle components.
If you can get - for the same launch cost - not 70 tons, but 1400 tons to orbit, even if they are in 54 ton, not 70 ton lumps - it starts being really questionable what the benefit of the 'shuttle derived' heritage is buying you.
I note also that SpaceX has an at least credible plan to get launch costs down from the above $1500/kg to $30/kg or so, in a totally reusable vehicle.
At this sort of cost, it becomes insane not to entirely reevaluate your lunar strategy.
For example, it may become entirely reasonable not to use a lightweight aluminium-lithium stir-welded composite structure which is indeed very light, but requires months of engineering to design and costs millions, but instead a half inch thick decent aluminium structure that costs tens of thousands.