It's very likely buried at the bottom of features since it's the thing that's likely to make some shy away even before they see all the good features.

Edit: In all honesty though, even though it does seem expensive, if you have 20 seats (20 developers), your payroll is already on the order of $1MM+, so $5k a year is almost a margin of error at that point, considering those devs use it all day every day.

Cut/Paste for the really lazy:

"Pricing for GitHub Enterprise starts at $5,000 per 20-user seat pack per year"