How best to spend gems?

[Schloss Ritter]

... besides save up a thousand and sell a sack, that is.

I'm looking to spend my gems to buy booster packs towards finishing my incomplete badges. However, I'm trying to use the gems as efficiently as possible on this, so I figured I'd buy boosters for ones that have the most expensive cards. Going through and checking badge by badge is proving extremely tedious when I'm only going to be doing this for one or two boosters for now. I've found a seemingly reputable third party site or two that will show me what badges have the most expensive cards, but I'm not seeing any way to easily cross reference with my current collection to only view the ones that I've already partly competed.

Thoughts? Thanks.

[Dendy]

Just sell them on market or trade them for games or sets