Install an APEXi S-AFC $700
Install an APEXi AVC-R $900
= $1600 (don't forget to add tuning costs... few hundred $$)
You can buy a PowerFC (full computer) for $1400 (already saved yourself $200) and you have complete control over the engine, and not just a bunch of "interceptors" that are designed to trick your existing ECU.
I'm currently saving for a PowerFC cos I can't warrant the costs/issues involved with piggyback ECUs.