It's only cheaper if you leave out the costs of degraded performance, higher turnover in personnel, and being less attractive as an employer. These are not easy to factor in, but the general gist of the last decade seems to be that the (unmitigated) open plan office has an impact on the bottom line. It's just hard to put a number on it, which you can do with concrete items such as floor space or the cost of adding a wall somewhere.
Construction materials supplier here. That is not necessarily true. There are two major drivers for open office plans:
1) The notion in architecture that open plans, by literally removing barriers, figuratively remove them and create a more open and inclusive office environment. This is a popular belief in the biggest architectural firms in the world, even though the principals at those firms all have luxurious private offices.
2) Recently some offices have flipped to open plans to accommodate hybrid working. E.g. allowing employees to WFH 2-3 days / week but then expecting them to hot-desk when they come in. This reduces the total footprint required, which can reduce costs...except many firms are using the saved funds to enhance the office in other ways (more natural light, better furniture, plants and fountains, catering, etc.) in order to attract talent.
Gensler has published a lot of this research and positioning publicly in reports and in their podcasts.