1350 tons of earth excavation for a single-family house

You’re probably confusing calculation by tons or volume.
About €18 per m³ is probably normal (with us).

[*]Excavate and dispose of 560 cbm of soil: 7,504 euros (13.40 euros per cbm) [*]Fill 720 tons: 7,128 euros (9.90 euros per ton)

I have now looked at our offers:

[*]Excavating and disposing of 560 cbm of earth masses: 7,504 euros (13.40 euros per cbm)
[*]Filling 720 tons: 7,128 euros (9.90 euros per ton)

We filled up with gravel, about 6

In Saarland and maybe in the Stuttgart area, the landfills are full. Thanks to the expansion of the A8 and the construction boom.

Hello,
so I come to 637.56 m² for 15.4 x 13.8 m (incl. workspace) with a height of 3 m (topsoil deducted, but you have to include the thickness of the gravel and clean layer and the slab) . Calculated with a compaction in the ground of 1.3, that already makes 828.828 m², and converted into tons with a factor of 1.7, you're already at 1409.0076 t. Multiplied by 13.5 Euro/t, that amounts to 19,021.60 Euro.

I also find the workspace very large. Are there reasons for that?
When filling up, I also wouldn't make compromises. It should be done properly (in my opinion best with gravel) and properly compacted. If you throw in some mixture there just because you have it left over, it can settle massively in the coming years. Then your nicely designed outdoor area sinks and you'll have to reinvest the money twice or three times.

The "refilling" then already includes installation? Otherwise, as mentioned, you can easily save half, and it's not a highly complex task, just laborious hard work...