A slightly cheaper option would be to get a bunch of Pi Zero W devices. They're $10 + SDcards. They would rely on WiFi which might be acceptable if the use case allows having a few dozen radios chattering next to each other. If not, then a $5 Pi Zero + $5-10 ethernet adapter might work.
Since the goal isn't to achieve maximum performance, the slow CPU & RAM (comparable to a 20th century desktop) might not be an issue. And the lower price would allow more devices so you could test more complex configurations and server management options.
Zeros are borderline unusable for actual compute due to having a single core and only half a gig or RAM. They make for great beefy microcontrollers and single-purpose boards, but are not the kind of thing where you want to run various network services.
(I have a cluster running Pi 2s, which have 4 cores apiece, but are still too short on RAM)
I actually have had a cluster of Raspberry Pi 1Bs before this current cluster. Though the specs are very much similar to a Zero W, the performance wasn't great.
I agree that it would cost much less, but I would also get much less utility out of it. At this point, the cluster is doing quite a lot, even the site itself is hosted on it.
My point here is that having room for future utility at not much additional cost, would be best.
Zero/Zero W at $5/$10 are promotional price, for first unit per person only. For second unit and after you have to order case kits or other high(positive) margin options.
Since the goal isn't to achieve maximum performance, the slow CPU & RAM (comparable to a 20th century desktop) might not be an issue. And the lower price would allow more devices so you could test more complex configurations and server management options.