What material costs should be expected for interior construction?

What costs, especially material, should one expect for the interior finishing,

As already written above:

Do not underestimate the time required.
Your relatives/acquaintances may be willing to invest a weekend, possibly even two. However, if they have to work on 4-8 residential units, this can lead to quite a lot of stress (between you, but also within the family of the acquaintance/relative).

You absolutely must ensure that the required materials are on site on time. If something is missing (even if it is just a small thing), everything may come to a halt. But do you also have storage space for the materials? Often it cannot be stored where other work is currently taking place.

If certain preliminary works are not finished as planned, the next person may not be able to start, which can delay the whole project indefinitely.
For example, someone would install the electrical wiring for you. They plan two weekends for this. However, delays occur in the execution or the person gets sick. As a result, the acquaintance who was supposed to do the interior plastering afterwards cannot start as planned. But he can only come after completing 4 residential units. By then, painting and flooring should already actually be finished. The relatives who want to do the painting cannot start due to missing plaster. When the plasterer finishes, those painters are then on vacation for 3 weeks and have other appointments on the following weekends. As long as the painters are not done, the flooring will not be installed (because it could be damaged by paint or ladders), etc.

What I want to say: you will need a lot of time. Every little delay/misplanning can come back as a boomerang in the end.

Do not expect too much from your relatives/acquaintances (in terms of strength and time).
For the painting and wallpapering work, my parents (>70), especially my father, helped. Towards the end, my father looked so physically exhausted that I (against his protest) took away the house key so that he could rest and regain strength. This was only partially successful (Dad just went shopping at the hardware store instead), but at least he arrived home safely later (570 km drive) and left the trees alone in peace ;).

It’s easy to overestimate your own capabilities under the aspect of "I want to save money" when it comes to own work.
We received the design for our dream house. However, the costs exceeded the limit. So we decided to compensate by doing some work ourselves. The list of own work got longer and longer. When we decided to install the drywall ceiling of the upper floor ourselves, we had a serious rethink. How do amateurs like us get large and heavy drywall panels up over our heads and screw them in with just two people?
At this point, we drew a line and reset the entire construction planning to zero. Our general contractor, who already had all the plans finished, discarded them again and then presented us (one after the other) with three more drafts. The last one became our new dream house. Incidentally, the GC said, "I was wondering how you were going to accomplish all these DIY tasks. But it’s not up to me to judge your skills. I can only give a hint here or there."

Lastly:
In my neighborhood, as well as among acquaintances, there are several who do a lot by themselves. I admire these neighbors. They work after every workday, every weekend, and live on a construction site for months. Some have already moved in although the screed wasn’t even completely dried yet. This requires great understanding within the family and lots of patience and strength. Comment from a neighbor: "You have to sacrifice your free time for something. Besides, I need my daily workout."