25 Rpi model B (for network) costs 875 $. Sorry but it is much more than a "regular" (linux) computer.
For this price, you could have a 6/8 cores (AMD) computer with tons of RAM, a powerfull cuda GPU and all you need around that (case, PS …). More interestingly, you could also have something like 4 dual core entry level systems (H61 + Celeron G530 + 2 Go of RAM).
For teaching distibuted computing, it can be interesting, for real applications it's a none sens, in terms of computing power and also most of the time in terms of power consumption !
If you're strictly speaking about costs, Raspberry pi isn't really high on the GFlops/euro scale. It is a device made for teaching, so it doesn't need to be high on any scale apart from the pricetag. Also, I'm quite convinced 4 desktop computers have so much electricity costs; it'll end up being more expensive than a whole cluster of raspberry pis at the end of the year. I'd expect raspi to live for at least a decade before dying (high MTBF, at least 100kH), while regular desktop computers are only rated for 3 years.
OP did not specify what for his plans were. If it is strictly for teaching grid computing; I'd use €1000 to build a 25~40-computer-cluster with raspis. If OP was planning on folding proteins, predicting weather patterns or whatever else may require extremely high GFlops; I'd just use the €1000 to buy an i7 or two because its GFlops/euro is enormous.
The mod mentioned that they designed a system with a very low energy-demand in mind (for whatever reason they did). Comparing power-saving systems to radiator-PCs is pure nonsense. A traditional supercomputer can heat up an entire cityblock, while a raspberry "bramble" uses less energy than my desklight