It would be foolish to expect Artifact's cards to ignore the all-governing market concept of supply and demand but seeing that Axe costs more than the game itself, regardless of how formidable the card may be, does make one think.
At the time of writing, Axe is selling at $21.63 and seems to be the most popular item on the Steam Marketplace. Note that the price changes almost by the second, albeit mostly in minute increments and during the writing of this piece it neither dropped below $21 nor exceeded $23.
We're talking about a 7/2/11 card that packs quite the punch, with many players considering it essential for a serious Artifact deck. The deadly combo of popularity and rarity obviously hits harder than Axe himself, as the jacked up prices are quick to prove.
Needless to say, adding new cards and tweaking them further means that Artifact's card pricing will balance itself out over time. Having launched only two days ago, the game is still in its infant stage and we've no doubt Gabe and Co will eventually get it right.
Unfortunately, Artifact's launch has been marred by monetisation issues, which even led to review bombing of the game on Valve's own platform. The bombers were enraged that Valve would charge $20 / €17,95 / £15.99 for the game and then make $2 card packs the only reliable method to get new cards.
Note however that if you're playing Artifact just for kicks, you're unlikely to run into the pay to win problematic that many players talk about. It's quite possible to grind out pretty much everything in time, even though some decks are likely to remain out of your reach for a while longer.
Although it may not be as obvious at first, this basically means that if you're looking to test your mettle in some high-level competitive play, you'll quickly find that Artifact doesn't reward cheapskates. Once Valve start rolling in additional content, it will get even worse because the theoretical amount of grind required to compete at a serious level will probably become too ridiculous to even contemplate.