Content in the marketplace priced at the 990 Minecoin price tier is listed as costing $9.99. It should cost $5.99, as shown in the image below when buying Minecoins separately. No other price tier is impacted. Buying an item at this erroneous price will buy 1720 Minecoins, contrary to the 1000 Minecoins that the button label says, leaving 730 Mineoins left over, contrary to the 10 left over that the button says.
Update: the issue has since been fixed.
The issue has been resolved as of Jan 24th.