From a fluff + RAW perspective, magicians have to compete with the average street doc with a bit of training and a decent med-kit. Healing magically and applying first aid takes exactly as much time. Stun damage takes an hours rest (call it your lunch break). Even in most of the stories, it only takes an hours relaxation for the mage to be ready to go after a serious fight.
SR4A Street Doc (page 291) has Logic 5, First Aid 4. A rating 6 MedKit is 600 nuyen, not too expensive for someone charging 100 nuyen per chummer (FastJack's table).
That means a DP of 15, average 5 hits for 3 Stun or Physical healed in a matter of 3 rounds (first aid rules on 252 - 253).
RC Paramed Shaman (page 142) Magic 5, Spellcasting 4, Willpower 5, Charisma 5.
Magical Healing DP: 9, for an average of 3 hits, 3 Physical healed. And it takes 3 rounds (Duration rules, pg. 203, SR4A)
From the rules for the Heal spell (SR4A, pg. 208), that's 1S for her drain. Drain Resist DP of 10, that's 3 hits (again) or 0S actual drain. She can keep doing this all day, baby.
So from a RAW perspective, it's obvious that the overhead is, if anything, lower for the mage than the mundane. Thus, prices can go down for the mage. Heck, the fluff of powerful magic being draining is also handled by virtue of not taking any drain (on average), this isn't powerful magic for even a modest magician.
The fluff issues, I and a slew of others have already explained. Mages are often from poor-sympathetic groups. More importantly, they have to make money, we've already seen that for short-term care, they are no faster than a street doc. They both get you patched up in an unbelievable 9 seconds. So really, how could a mage afford to charge more than an equally skilled street doc for the same service, at the same speed, for the same quality, just with less overhead, fewer cleanliness requirements, etc?
They gotta compete, their prices are gonna be competitive, they can't offer much else (strictly speaking to healing itself, atm).