I am working on a one-off home automation project for personal use. The software is not in question (i.e., I can write the software to drive pretty much any kind of hardware out there).
I will have 2 or 3 physically separate, networked (via Wi-Fi) nodes, installed in fixed locations in my house. Each will access a shared server (already exists) via Wi-Fi, and accept input from the user via button/touch input.
In terms of interface, I could get by with a 16x2 character backlit mono LCD screen plus a couple of momentary contact buttons. A small touch screen + web browser would work just as well. I also need some kind of sound output. A simple buzzer/beeper would suffice, but an actual computer audio interface and small speaker would be acceptable as well. Among the options in this paragraph, I don't really have a preference, so it will come down to cost.
Size footprint isn't terribly important, but think wall-mounted panel, not computer workstation.
I have considered things like the Raspberry Pi + 16x2 LCD + Wi-Fi + buttons, which would run me about $100 for my minimum requirements. I'm hoping to find something cheaper than that, but I won't put a number on it that might be unrealistic.
Operating system is a don't care, as long as it supports the required hardware. I can program a µc if need be.
I've also considered touch screen devices such as used iPod Touches. Jiggering a web interface for my software is already planned. In general, I don't mind buying used hardware if it's likely to be reliable and give me at least a few years' life.
I will put the software on my mobile phone as well (as well as my PC), but I still need the fixed installations as I do not usually carry my mobile phone around with me when I'm in the house.