Interior plastering cost too high?

Hello dear forum members,

I have a huge problem with a craftsman's invoice and need a bit of background knowledge.

What is the maximum cost of lime plaster per square meter? My area is 435 sqm.

How many hours are roughly needed for such a job?

My craftsman has now sent me an invoice just for the base plaster of €12,200 - working time 330 hours. Material costs €5,000.

I have already informed myself through my lawyer that this invoice is much too high, but I would like to settle out of court. I think that the working time was simply doddled here.

due to working hours:
I have about the same area, if I remember correctly 150 working hours (3 days 4 men + 1 day 3 men) lime gypsum plaster type Röfix 150 and about 80m² in the wet rooms cement gypsum plaster röfix 180.
are such works not usually billed per m²??
if you take usual values, values up to 25€/m² are quite possible, that would be about 11,000€.
which plaster type do you have on that??
but when I look, material costs also seem high to me. with 15mm plaster thickness you need about 16kg/m², a ton of the plaster I use costs about 320€ (in Austria), I think it does not differ much, about 400€ more comes for the silo, you are at 2640€. Corner beads cost 0.75 per running meter, I think all in all with a similar construction method as mine you end up at 3000€ material GROSS list price.
just looked: according to the price list all plasters are in the range of 280-380€/ton, so the material costs also seem high to me.

What was on the offer?

I got the KIP from Schwenk and the MEP-it in the wet rooms. According to the delivery note, 10 tons were delivered in the silo and 3 pallets. I find the material costs more or less okay. I think the working hours are the big problem. 330 hours for the first layer is simply too much. I had an offer for the lime gypsum plaster, afterward I was advised to use lime plaster because it's better. Costs are only minimally higher.

According to Schwenk, approximately 12.5 kg/m² is consumed per 10 mm of plaster thickness. So, depending on the stone and plaster substrate, the 10 tons estimate is quite accurate. Material is also quickly lost during machine startup and cleaning after the spraying pass. However, the working time is really very high. But why billing by the hour? Especially with plastering such large volumes and in new construction? It is always billed by m². Prices of up to €50 per m² net are also possible. It is also a question of substrate preparation and the desired appearance. When billing by the hour, a daily or weekly report should normally always be written. This way, the work performance can also be better assessed.

I had an offer for the lime gypsum plaster, afterwards I was advised to use lime plaster because it is better. Costs only minimally more expensive.

And what was the offer amount in euros?