Slight slope - Should you fill or build on the slope?

Is not even either.

Where is the access for both properties?
What are the cardinal directions?

Can you agree with a basement or ground floor that goes into the hillside at the back?
The outdoor area can then be adjusted, possibly also with a slope without retaining walls, etc.
Each floor has garden access

So I'll put it this way. Both plots cost only €30,000 each. There is no broker fee. I just needed to know approximately what additional costs would arise when building on the slope.

To the point:

"There is no cost-effective realization of house and yard on a hillside property"

The additional costs can only be planned/estimated once you know how you want to work on the slope and have a quote; not before.

Well, I'll put it this way. Both plots cost only €30,000 each. There is no broker's commission. I just needed to have an idea of what additional costs would arise if building on a slope.

No one can tell you here without exact information: Except that the measure will be more expensive than the plot itself costs. That doesn't help you if you plan with €30k and it turns out later that transportation/disposal will cost an additional €15k because the soil class is very poor or you have to compact again with special or more gravel.

Depends on what you want and especially how steep the slope actually is

the less earthworks the cheaper

basement as an additional floor or a habitable basement

it costs more money, how much no one can say

Do you already have a developer or architect? They should be able to estimate it reliably.