Because you can make a tattoo of a power with target "Creature Touched," you just can't have a tattoo that will affect another creature (thus, your buddy could not benefit from your tattoo unless he was wearing it).
The reason that Personal spells are more expensive is that, design-wise, they are supposed to be limited to the caster classes. Thus, giving a Wizard a fairly powerful combat buff as a 3rd-level spell with range Personal is more or less balanced, because it is unlikely to ever be able to be used by someone else.
EDIT: I can't seem to find the text regarding a cost increase in the 3.5 SRD at Wizards. Is the cost increase a 3.0 thing?
The base price of a tattoo is otherwise equal to a potion, which makes sense.