As per Schwalb's article Henchmen and Hirelings, there exist level appropriate henchmen at every level. They are, effectively, minions and priced like consumables of their level. Given that they die in one hit, that's about the right pricing model. A useful houserule could be that they are rendered unconscious in one hit, rather than diedie, and should probably price them as normal magical items of the appropriate level.
I recommend the use of hirelings in moderation. While they are excellent RP material, only players comfortable with tracking multiple locations on the board should engage with them: They are less work than an equivalent animal companion, but the necessity of tracking opportunity attacks and other out of turn attacks can be tricky. On the other hand, given that they get a free move and standard every turn, they can be a quite significant ablative meat layer that provides better "reward" than a level appropriate sword. Hirelings expected to take additional risk are valued at 2x or 3x the cost of a potion. (Don't think about this too hard in terms of "real" economics. Just.. )
In response to the tracking issue, given that hirelings act after the character's initiative count, have very simple powers, and 1 HP, they do not add much bookkeeping to the game for an experienced player.
Henchmen, on the other hand, use DMG 2 rules and are statted up as not quite full characters. I'm not entirely certain they're worth giving as a reward. They feel like they're more useful as cardboard cutouts replacing absent characters: they have a persistent presence at the table. If you're giving hirelings as a reward, use the item price (not potion) price, and make sure they can get back up after the battle's over. It's the GM's job to roleplay them outside of combat, and (if done well) they can take parts not dissimiliar to NPCs in KOTOR (the computer game) where they generally contribute to combat in a "yeah, sure" kind of way, but have lovely characterization. You may also want to adapt the artefact subsystem and give them additional abilities depending on how much they like the characters.