A complete static calculation for a "typical" single-family house roughly costs 6 to 10 k according to HOAI - so I don't find 400€ too much for an assessment. Offers with hourly rates for engineers in general (so not HOAI/construction) are usually in the range of 90 to 120€, for absolute specialists even daily rates of 2 k and more, so I find 400 EUR for the assessment of the technical feasibility of a project worth several hundred thousand EUR well spent money.
Of course, static calculations are a complex field for which we ultimately will pay accordingly. But this is not yet about the static calculation of a single-family house, rather only about a trained eye and thus a rough assessment. I do not expect a structural engineer to spend half a day on this and already calculate some examples. The architects & structural engineers (timber construction) we visited gave the impression that one can read that off in 2 minutes. If it doesn't go that fast, then sorry that I asked again here.