First I just want to say that, coming back to MTG after 12 years, this site is freaking amazing. It beats the he11 out of every automated/scripted spreadsheet we ever hacked together back in the day, and I can't thank you enough for making it free. Cheers to you, my friends.
Now to business: I would like to propose two updates to the way mana costs are handled.
1) Filtering: This is more of a semantics issue than a bug. While browsing Magic cards via the Advanced Search, when I filter by Cost the option names are "Is smaller than" and "Is largen than" (that typo exists in the UI, incidentally). If I set the filter to "Cost is smaller than 2" I would expect to see only cards with a casting cost of 0 or 1; however, cards that cost 2 also appear in the list. I would propose that the options be changed to "Is less than or equal to" and "Is greater than or equal to" - that way it's only a UI change and no logic needs to be updated, but the law of least surprise is followed.
2) Sorting: Right now, when I sort my decks by Cost, cards seem to get sorted by their converted mana cost without any regard to the number or types of colored mana they take to cast. I don't know what the TCGPlayer data looks like, and it could be that other players don't care about this sort of thing, but when I'm considering the cost of a card, I consider colorless mana to be slightly less expensive than colored mana. In other words, if I was sorting white cards with a converted mana cost of three the order would always be: WWW > 1WW > 2W. And indeed, when you get into multicolor cards, the more colors required, the more expensive: BWG > BWW > WWW.
These are just my 2¢ on what is already a great application. Please keep up the great work! 
-decompiler-