The real strength of the camp when initially build should be to quickly churn out those Exiles, and provide a short buff that allows for dynamic decision making. 90 seconds is indeed too long. 20-30 seconds sounds reasonable. Either a longer duration and a weaker buff, or the other way around. As for the price, I want to say that I would not want to see it that much more expensive, since the charm lies in its cheap price. I think 600 is reasonable. That way, you have to consider, Stables or Exile Camp?
I also agree with what Mogat said about the exiled mechanic. If they could be recruited from the outpost, which is cheap and not impossible to defend because troops get a speed buff around it and it has defensive creeps, the risk factor would be drastically diminished.
I mean, yes, it would make sense, but it will screw over the mechanic and its uniqueness, imo. You get the massive 50% damage boost, which allows Rohan's heroes to deal with many of their more expensive counterparts, especially in 1v1 duels, but the downside should remain as substantial as it is now.
Although, Eomer in the Exile Camp sounds like a good idea, because he formed the Exile Camp, based on his well...exile, haha.