Even on T1 frigs!
I have a Dual Rep Merlin that has to use a T2 powergrid rig (ACR) to fit its massive tank and collision accelerator. If I use Ions the fit does much less damage and the additional cost of fully upgraded midslots brings it close to the cost of the T2 ACR (it does about 30 less DPS, has 1500m less range with Null, has a little better tracking, and goes much faster with T2 AB and T2 web/scram or double web). I found out that if I compact my Monoprop AB and use an 8 mil 2% PG implant, I can use a T1 rig and reduce the cost of my fit by 4 million. This means that if I lose two Merlins for every pod I lose (already a really unlikely situation to lose that many pods), I will about break even.
In addition, this implant allows me to upgrade Afterburners on some other fits, add damage rigs when I couldn’t previously, upgrade guns, and even use that Polarized Slasher!
In short, PG implants are fantastic. Get yourself some and have fun making your killboard a little more green!