Another cheap option are Fujitsu Futro's. They're meant to be thin server clients meant to operate larger server racks (I guess?). Anyway, they come with 4-8GB RAM, an SSD (most people upgrade them), and even have a PCI-e slot (depending on the model) to use with a 2.5 or 10 Gbit ethernet card, for example. Around $50 on ebay.