how much to fix ball bearings in bike?

[calimachos]

i dont know too much about bikes, but the other day these little metal balls started falling out from the middle of my back wheel and now it whobbles too much to ride cuz its almost falling

i just wanna know anyone out there would how much a mechanic could charge for fixing that...i know it would vary, but a reasonable range in your expirience

thanks!!!!!!!!!!!!!!!!!