While it sounds simple, I believe it would require quite a bit. You likely pay different amounts each time you get a card. Then, when trading, if you acquire a card as part of a larger trade, if the trade is not exactly 1:1 on value, you'd need to calculate the cost of that card coming in from your trade.
On the back end, there would be much more storage required by the folks at Deckbox (potentially, every card having a unique entry--though not all would likely go to that level of detail). Storage may be cheap, but hosted storage is not necessarily so, especially for a free (ad-supported) service.
If you have multiple copies of a card that you purchased at different times, when it comes time to trade, do you want to look at each individual cost, or some average of them all?
I keep track of my overall spend in my personal accounting software (e.g., MS Money, Quicken, Mint). I just create a category for Hobbies: MtG. Then I know how much I've spent in a given year, and I can compare that against the value of my collection (and how much it changed during the past 12 months).
I'm not suggesting it is impossible, or that Sebi will nix the idea, but am just the type of person who often sees beyond the immediate request, and thinks of the long-term impact.
Cheers!
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TradelistBlack and Blue--not just for bruises anymore.