You need to first look everywhere and find as many as you can, and then go by condition and price.
I posted on glocktalk for some in the ten ring, asking where to get them. I had more than I needed for around 100-125 all day. In good shape too. I wound up with a pretty unused 20 with no fingergrips or accessory rails and copper lube on it. And seven 15 rounders with stickers still on them. It was cool taking the stickers off because there were prices on them and they went down as I went deeper. I bought the package and figure the mags ran around 100 bucks each maybe a little more. All of mine drop out of the gun fully loaded, I guess due to the amount of plastic on it and the fact they are new, but rather old.
The 45 and 10mm are the hardest ones to find. The 10mm made all my others for 9mm and 40 *&* seem like finding a gallon of chocolate milk at the store.
If they are beat up, price goes down. Mine are/wear mint. I bought them to use not keep and sell later.
Now, about that durn grams +5 extension. That makes you wonder.