Buying workers is the best possible deal.
They come with the limit of work at half rate, but You ALSO slow down the AI improving it's territory. For abou t $25 I can speed up my land improvement, and slow down there land improvment. If this isn't a bargain, what is?
They show up at the capitial, and I often use them to start a project in another direction. A city not on the road network, and the new worker goes that way. Every little bit helps.