Well, I just noticed that you have been filling in "Unknown" for the sell price of cards that don't have one yet. I mean, you do it along with other edits so I know you aren't just fooling around. However, I thought I would let you know, in case you didn't already know, that the value "unknown" is already the default and appears when that value isn't specified. Hence, although perfectly okay, you don't need to set the price to that.
I know, I am just saying that records show that you have been setting that text value to "Unknown". Additionally, that is done by default if there is no given value so explicitly putting it in isn't neccissary. I wasn't sure if you knew that already.
For instance, I see that you recently edited Episode Shockwave 2. If you go to that page and open the editing form, in the box for selling price, it says "Unknown". If you were to delete that value, leave it blank, and save, the page would still display the value "Unknown" because it is in the template code to use that whenever a value isn't specified.