Looking over the costs of mounts from the 4E PHB and Adventurer's Vault, one finds that all the mounts - with the exception of the level 1 and 2 ones - are priced like magic items. Often items at higher levels than the monster's level.
This is quite understandable, as many of these mounts grant powerful abilities. The Rage Drake, for example, give its rider a +2 to hit and damage, which stacks with every other plus. Neato. A Dire Wolf, on the other hand, is just a dire wolf with no frills, so it costs the same as a magic item of its level (5).
The interesting part is extrapolating this to henchmen. A mount shares its actions with the rider, so it's not an extra set of actions on the field, just a power boost to the rider. Buying a henchman, on the other hand, would mean that there's an extra ally taking actions every round. Which is a huge boost.
But let's pretend for a while that it works. Many mounts cost the same as a magic item of its level+2. Applying the same reasoning to henchmen would let you hire a bandit (Human Bandit, level 2) for 840 gp (a level 4 item). Mind, this is a slavishly loyal bandit. For the cost of Bloodcut Armor +1, you get an ally that flanks with you and can unleash a Dazing Strike once per encounter. That's... a bargain.
Weekly rates make more sense than "buying slaves", since your adventurers will soon outlevel the henchmen. One might want to halve the cost and make that the weekly wage. Or if you find that the Bandit is indeed even better than an equivalent magic item, make the original cost his monthly wage - he'll eventually quit.
Finally, companion characters from the Dungeon Master's Guide II are probably better balanced as PC allies than creatures from the Monster Manual. PC and monster numbers are slightly different, after all.