Ex Calling is pretty awful all things considered. It's a weaker version of IE with a negligible passive for it's only reward upon completion, which often just translates into 2.85 damage to base regen over 1.5 seconds end-game.
Considering most items give free stats for completion the more expensive they get, having 1.9 extra DPS is pretty lame. IE gives more than the 250% crits for free, tossing in some AD or Crit chance in the mix.
Atma's, Statik, and especially Youmuu's are all better crit items with better stats for less gold if you're looking for Avarice upgrades.
Season 2 it Calling was freaking awesome and had amazing synergy with GP.
18% life steal, 15% crit, GW on an 8 second duration, 20 sec CD cooldown active, all for 1350 gold. You could grab 2 for 30% crit and 36% life steal, which was completely broken unless you ran out of inventory space (but still worth).
Now though - you should really stop getting Ex Calling and get something else. Statik will cause GP to shoot lightning out of his gun. It makes his gun shoot lightning - how cool is that?
Also, Youmuu's builds out of the massive early-game power-spike item Brutalizer along with bankplank's avarice blade, giving enough insta-ADC stats in Youmuu's you can actually feel like a solid end-game critplank with only 1 major item. Also gives you a lot of flexibility in going Tankplank, as you've got all the key critplank stats already, so things like Randuin's or Warmog's can do a nice 1-2 punch of offense and defensive itemization.
Atma's also has pretty obvious synergy with tankplank builds.
Ex Calling is a complete waste of gold, especially considering you could have kept it as an Avarice Blade and just bankplanked until end-game.