Check out the Futro S740. It is more powerful than the pi, uses comparable power and still quite compact.
They can be found (in Germany at least) for 40€ with 4gb RAM and about 50€ for 8gb of ram. Ram is upgradeble, so is storage.
If you want something (much) more powerful, there is the Lenovo tiny line, for example the m710q or m720q (one cpu generation newer).
There are machines (like the Futro s740) that can be powered by POE as well.