Some quick pizza box math. Saturn 5 mass to LEO was about 130 ton. It could bring the entire CSM, LM, fully fueled, all the way to LEO and additionally to trans-lunar injection (free-return trajectory around the moon). Starship, although it has not demonstrated (yet) a heavy lift to LEO, is theoretically capable of 100 to 200 tons to LEO, and you get the booster and ship back to fly again with days, or hours. Starship block 3 dry mass is about 100 tons, so there is either your ENTIRE mass to orbit at the low end, or HALF at the high end. All of the LEO test flights to date have essentially arrived to a modified orbit with empty tanks. The first operational LEO flights of Starship will probably be closer to having somewhere between 20 to 50 tons usable payload to LEO with full re-usability (while they're dialing it in - Falcon 9 basically doubled it's mass to LEO from when it first started flying to now). Wet mass, or a fully fueled Starship is around 1,000 to 1,200 tons. Which means, if you want a fully fueled Starship in LEO, you have to bring 1,100 tons of propellant, at 20 to 50 tons per launch (eventually it will be 100 to 200 tons). Solve for how much delta-v you need to get Starship's extraordinarily big ass all the way to the moon (and back), and you can figure out how many propellant launches you need. SpaceX estimates this will be 8, or 20, or whatever, depending on which numbers they are using.

One other point on the cost of launches - The SLS system costs a staggering $4 billion to launch, and only the SRBs (sort of) and the capsule are "re-usable." Starship, in fully re-usable configuration, will cost $3 million per launch (per SpaceX). Let's give ourselves the benefit here and say it costs $100 million to launch a Starship (about twice what F9 costs today) - that is 40 times cheaper than an SLS launch. So even if it takes 20 propellant flights, that's still half the price of an SLS launch. More likely, it will be 100 times cheaper (tonnage to orbit) than the closest competitor anywhere in the world. In 10 years, high school kids will be launching satellites for science fair projects.