Market value is $25, and seems fairly stable right now as well, albeit the foil version fetches 10x that, with listings up to $250 at TCGPlayer right now
Mirri, Weatherlight Duelist [Cat]: The first clause is limiting, but not that limiting
Konamis approach to rebooting the physical card game was a more faithful interpretation, following the rules introduced in the manga as much as possible
Perhaps Erase is a good card against this particular mono-red deck, but that would only follow from logic along the lines of: "This matchup is very favorable for me in the absence of Outpost Siege, so it's worthwhile to sideboard a narrow answer card because it prevents one of main ways I can lose." Such specific and thoughtful analysis of the matchup is great, and it might lead you to play with one or more copies of Erase in your sideboard
and sometimes commons go up because a new deck starts running them and the people who have them aren't willing to sell because they are worth so little