So ... according to
@mike wightman ...
These are my opinions, which are as valuable as anyone else, more so to me personally.
- Living in space is NOT expensive ... because, Space Magic™ in the 57th centuryyyyyyyyyy!
It is orders of magnitude lower than it is today, which is the point being made. And yes it is due to space magic with regards to artificial gravity, not so much with regards to fusion power (likely to one day be achieved... 50 years time...), and environmental control (possible now but much cheaper and reliable in the 57th century)
- EVERY location in Known Space has more air, food and water processing/manufacturing than they know what to do with such that the expenses associated with life support in space become so marginal that you need to use calculus in order to approximate the marginal costs (to somebody).
That is not the argument I am making. Everything has a manufacturing cost, a transport cost, a labour cost a maintainance cost etc. These costs must be paid, but here is the kicker, they are orders of magnitude cheaper than they are during the early space flight era of Terra.
- Fusion power is "cheap" and "unlimited" (cited 2x in the quote above
) rendering the costs for ... well ... ANYTHING, really ... so low that the credit stops being a unit of value (and more a matter of "keeping score") at which point capitalism is dead, because ... superabundance.
- Yet strangely enough, fusion power is neither "cheap" nor "unlimited" when it comes to the construction of starships, for some unknown reason ...

Others have already answered this. Fusion power is cheap, and as to unlimited, there is no cap on power plant size in HG is there...
If that's true, why are credits needed to pay for ... well ... anything, really?
Because the Imperium is not a post scarcity socialist economy. Only certain worlds are allowed to develop TL15 infrastructure, only certain worlds are allowed to be manufacturing hubs, they don't even allow every world to have TL12 infrastructure which could be easily achieved.
With "cheap unlimited fusion power" and makertech, the only scarcities possible are artificial (and therefore, coercive).
This is the conclusion I have reached about the Imperial economy too.
What purpose do credits serve, when because of "cheap unlimited fusion power" there are no limits on the extraction and processing of resources (animal/vegetable/mineral)?
Because the wealthy and powerful want to stay wealthy and powerful, they don't want utopia they want dystopia for the masses that way they can maintain their wealth and privilege.
And just for fun, I'll add this from LBB3.81, p19.
Apparently, "life support" isn't "free" even for people who live on planets.

Who knew?
So people have to spend money to buy food? Note that once again these are the costs for a consumer society...