We built an all-in for under 300,000€ seven years ago. The plot of land was included in that with about 45,000€. For the rest, there was a 125m² KFW70 house including a double steel garage and outdoor facilities on flat land. At the time it was feasible for the price, it certainly could have been a bit cheaper if we had stuck even more to the standard. At least initially, we watched the budget carefully to not exceed it, and of course, there is no controlled residential ventilation, no electric shutters or similar included. Towards the end it was relaxed, as we were able to save some additional money due to the long construction period.
However, there has also always been around a 5% price increase in recent years. At 700x100€ it would already be significantly more than we paid. I personally doubt that it is still possible to manage 300,000€ for the plot without a lot of own labor.