Just noticed that CNC machines are getting to be cheap as well. A sample guide to CNC machines looks at how they can be used in conjunction with moulding techniques to fabricate moulds for plastic parts as well as produce metal parts.
These CNC machines are not quite as cheap as the 3D printers, but they are in the ballpark - plus if you create the moulds correctly, can be used to scale up small scale manufacturing of plastic parts much better than you could with a 3D printer.