Base plate on the top edge or bottom edge? Who has experience?

Depends...
Excavation here, for example, is the worst possible (clay, silt, sandstone chunks, ...), so only suitable as fill material "at the very bottom." Therefore, quite a bit of good soil needs to be brought in.

As a rough estimate (in the countryside, short transport distances): screened topsoil approx. €25/ton officially. On the short service route approx. €300 for a tandem load (22 tons). But that only gets you the material free at the construction site, not yet installed. That means your fill can possibly become really expensive; in any case, compare it to a basement. Then it depends on how good your soil is. Do you need larger quantities of good soil anyway or do you rather have some left over?

If you want to calculate, just take the expected area you want to fill and the average height to be filled and calculate the volume (m^3) * 1.8. Then you have the required tons for compacted installation...
In the end, it also depends on your ideas for the property. Do you want it flat like a baking tray, or are you more the garden type who would like to terrace it, possibly with a small staircase and dry wall for planting...

A basement does not change the fact that the area around the building must be backfilled, and if the excavation material in the construction site is no good, it must be disposed of and the backfill material must be purchased. The basement then saves nothing again.

You can quite largely follow the natural terrain with the garden layout. It just won't be football turf but something else, but it can turn out really great.

It is also important whether there is a ridge height restriction in the development plan for the area - and what the reference point is (street or original ground level).

I actually see the slope only as a wall on the other street... are there now 2 rows of properties in the middle or just one? From where is the property accessed? I can't tell whether it's 30 or 60 meters deep. The wall back there could also become public land... One can guess and assume a lot.... the seller certainly has quite a bit of information in their pocket