Neighboring plot divided. Possible?

Where do you think the limit is, from when can one build, from 300000 or 400000... or from 5000 net per month?

I have no idea that there should even be such a limit, and certainly not one based on an amount. That does not prevent me from considering it worrying when prospective builders preferably want plots that leave more than 3m distance between house and fence only in the south. Floor area ratio limits are necessary to protect the water cycle and other environmental aspects; they are not a tool of envious millionaires who would deny ordinary workers their own home.

you should always look at things in terms of percentages. In my opinion, the monthly installment should not exceed 40% of the total net amount. Better somewhere between 30-35%.

I also see that as problematic. As soon as an investor is involved, the area is maximally utilized and in the current situation, those wanting to build or buy are rather glad to have any option at all. They take what is available. The investor and the city profit from this, since the fees are based on the total value and not just the land. Of course, the counterargument is that if more people move to the countryside, the plots are still sold normally (without investor). For us, it was mixed.

As soon as an investor is involved, the area is utilized to the maximum

Usually (i.e. for four residential units it is still like this) the appearance of an investor - here somewhat simply called "developer" - does not change the ratio figures for the development floor area ratio and the floor space index. And as I already wrote above: especially private buyers build with the highest possible density.

The investor and the city earn from this, since the charges apply to the total value and not just the plot.

I do not know the distribution key of the property tax - but I doubt that a municipality would have an active interest in preferentially allowing plots to be developed by developers. In the semi-detached and terraced house sector, developers simply handle the easier processing of construction. Municipalities are rather interested that the residents of the new development areas do not nag them forever with the question of when the final road surface and streetlights will finally come.

Municipalities very often have an interest in densification - not primarily because of the fees, but also with new residential areas because they have to create compensation areas. In the handling, working with a project developer who builds several units at once results in significantly less effort. That may occasionally also play a role. Ultimately, as we see here in the forum just as writes, the finances of many homebuyers simply no longer suffice for larger plots in the suburban belt. As a seller, however, I want to get rid of a plot and not divide it and have effort. I would rather sell to one than to two or more.

I have also written about this elsewhere...
In front of the children’s school, eight building plots were sold by the city; there used to be another school building there. First, one half was sold, where there are now four single-family houses. Then the other half (here the street probably had to be officially designated first). Here stands a top-class single-family house, everything was ready from the start, and there are two off-road vehicles with a star in the garage. And a developer acquired the other three plots and built semi-detached houses. There really is only the setback area maintained, with just enough length at the front for a parking space. None of these homeowners needs a lawnmower.
The state can of course collect a lot of real estate transfer tax this way; each semi-detached house half cost 400k€. Surely the city also collects more property tax than with the single-family houses...
Another plot, in my district and actually just right for a single-family house with a nice garden, was now built with two single-family houses. Here too, setback strips and space for cars in front, nothing more. I think two parties joined forces here; there is no developer behind it. But financial reasons surely also led to this.

I do not know the distribution key of the property tax[...]

The property tax is levied by the municipality and is used only by it. Although the trade tax has a levy, the property tax does not (directly). Through collective municipality or district levies, the money is then still somewhat "reshuffled" ;)