$3k pays for a 1U server with a 32 core 2.6GHz Epyc 7513 with 128GB RAM and 960GB of non-redundant SSD storage (probably fine for build servers).
All using server CPUs, since that was easier to find. If you want more cores or more than 3GHz things get considerably more expensive.
Not that they are bad and would not be way better than what they have, just that I though the parent was quite the optimist with his Zen4/Zen5 pricing.
Then there's also the overhead of setting up and maintaining the hardware in their location. It's not just a "solve this problem for ~$2,000 and be done with it".
I don't know the actual specs or requirements. Maybe 1 build server is sufficient, but from what I know there's nearly 4,000 apps on FDroid. 1 server might be swamped handling that much overhead in a timely manner.
Space in your basement or the colo rack of a datacenter along with power, data and cooling is an expense on top. But whatever old servers they have are going to take up more space and use more power and cooling. Upgrading servers that are 5+ years old frequently pays for itself because of the reduced operating costs (unless you opt for more processing power at equal operating cost instead)