In the 3.0 DMG, there was a sidebar saying something along the lines of "You could offer magic tatoos, which would have the same benefit as some other item but cost twice as much and not take up a slot." That would tell me that there are two ways of handling the tatoos:
1: Tatoos do not take up a slot, cost twice as much (i.e. 32k for Dex +4).
2: Tatoos do take up a slot, and cost roughly the same (plus a little for not being stealable, minus a little for not being replacable with some other item - probably a net plus though).
A slighlty different version would be to use a psionic tatoo of animal affinity with the Permanent Tatoo feat (from Hyperconscious) applied. It would be able to be activated once per day, giving +4 to Dex for 3 minutes. A permanent tatoo doesn't cost more, but it does take up four tatoo slots (out of 17). The prereqs are: manifester level 7, Scribe tatoo, Enduring tatoo, Craft (tatooing) 12 ranks. Enduring tatoo, in turn, is another feat from Hyperconscious that let you use a tatoo twice (with at least 24 hours in between), taking up two slots. Prereqs: manifester level 5, Craft (tatoo) 6 ranks, Scribe tatoo.