Fairly cheap per hour. I think it realistic to be less than 25 cents per hour. I suppose it depends how much effort you put into detailing, and playing.

How much it is per hour depends entirely on how often you play it. A Warhammer army can easily cost hundreds of dollars, and over $1000 is not unheard of. It can be cheaper, but RPGs can be completely free and eat up as much or as little time as you want.

Though I suppose if you count the time spent painting the minis as time spent playing, it's likely to end up fairly cheap per hour.