The cheapest smartphone with an existing Mer/Nemo image seems to be the N900. I got mine for EUR 69,- on eBay. Technically it's ok, but the color has been scratched of some of the keys on the keyboard (got a new keyboard cover for EUR 17). The N9 I bought is brand new and was EUR 269,- (the black 16 GB version). Once I know Mer/Nemo a bit better, I'll try installing it on the N9.
Oh, and I forgot the Galaxy Nexus in the list of devices. Would that be a good device for testing?
