Historically polders were paid for by private investors or feudal lords; who then owned the land. The reclamation of Flevoland, was carried out and paid for by the Dutch state but the end result was the same: it then owned the land. In the case of newly established villages, the housing plots were sold by the local municipalities to individuals or housing corporations.
The newly gained farmland was a different matter. At the time of the land allotment, the farms had already been built by the Dutch state and each one had a specific number of hectares assigned to it. The Rijksdienst voor de Ĳsselmeerpolders (National service for the Ĳselmeer polders) was formed and put in charge of assigning the farms in 1947.
Anyone could apply to become a tenant farmer (this is important, no new farmer would really own his land) but the Rijksdienst had extremely tough selection criteria for the would-be farmers. Initially, it was said that priority would be given to the day laborers who had actually worked on the land reclamation, but in the end, those interested were required to prove sufficient agricultural knowledge, financial health, had to be church-going and socially active in their communities. Those interested would fill out very detailed forms, which would then be checked by government officials.
One would expect only the first two aspects to truly matter, but one has to remember that the Dutch government was building everything from absolute scratch; there were no existing settlements (apart from those previously located on a few tiny inhabited islands) and hence no social infrastructure. Religiosity and social involvement were therefore considered to be very important.
More than ten thousand people applied for the farms in the Noordoostpolder, but only 1700 were found to be sufficiently qualified to even be considered. They (and other applicants) were then shown around the farm; after which they would receive a message from the government informing them if they had been selected or not.
The same process was repeated in the Eastern Flevoland polder, though halfway during the allocation of the lands there; the government stopped the allotment to relocate farmers from North Holland whose farms had been requisitioned for the expansion of Schiphol airport.
2. Het nieuwe land: een verhaal van een polder die perfect moest zijn, by Eva Vriend (2013)