Being able to deal damage equal to the number of Mountains you control to a creature or player makes the Spawn's effect versatile. He can take out a problem creature, or deal a substantial amount of damage to an opponent. While this isn't necessarily an efficient way to spend 7 mana in most Constructed decks, in Commander, you're not going to mind paying 7 mana to deal a ton of damage late in the game after you've played most of your Mountains.
Two Commanders that take full advantage of Spawn of Thraxes' ability are Feldon of the Third Path and Zirilan of the Claw. Feldon can create token copies of any creature card in your graveyard. This means you can create a Spawn token for only 3 mana as many times as you want, getting that effect and also getting a 5/5 with flying and haste for that turn. Zirilan can search him right out of the deck for three mana, and in this case he also gains haste. However, he has to be removed from the game at the end of the turn. In both cases, one summon is almost always going to be worth it.
How much is Spawn of Thraxes worth? Being a promo that had a high supply and relatively low demand, Spawn of Thraxes' price is probably never going to exceed much more than $1. Still, this Dragon is a great addition to many Red Commanders' toolboxes. He may not win the game on his own, but he can deal a great amount of damage for often as little as three mana. If you're sorting through bulk bins and happen to find a copy, Spawn of Thraxes is worth picking up on the cheap for your collection, especially the foil pre-release promo version with alternate art.