Creating anime costs a lot of money. I admit that the numbers in that answer are more than likely ballparks, but you get the gist - per episode numbers are in the neighborhood of 11M yen (or ~$97,200).
Add on top of that the work that has to go into localization for release:
Certain scenes may
have to be omitted, cut, shortened, censored or removed due to the difference in what is acceptable to broadcast in that country. (I'm thinking of DiC for their creative dub of Sailor Moon, and 4Kids for just about everything they touched. There were also scenes from various other series that had to be removed; you can read more about that over here.)
Certain jokes just don't translate well, no matter what you do.
The cost to do this adds on top of the original production costs, since there is the very strong likelihood that it's not just voice actors and actresses coming in and doing the work.
Above all, the series actually has to be doing sufficiently well in order to earn a dub, or there has to be sufficient demand to deem it profitable to do one. One of the best examples that comes to mind is Toradora!, which had its original version broadcast and released back in 2009, and had its dub come out about five years later. It's safe to say that yes, there was enough demand for this series to come out with a dub.
Suffice to say, the cost to produce the non-dubbed, often subtitled series is sufficiently less than hiring more staff to do essentially the same thing. This is why dubbed anime is definitely at a premium - because the work being done is a premium service for non-native Japanese speakers.