i've had one of my 2 torsion springs break twice since 2001. it's barely a 2 car garage with a single door. without springs or counterweights, they're heavier than you'd think.
nice thing about having 2 springs is that you can add enough tension to the remaining spring to lift the door until you're able to replace both of them.
1st time i made the mistake of ordering a beefier spring, thinking that would make an easier job for the opener... wound up not having enough oomph to close the damn door... get the same size springs!
not a bad d.i.y., just be very methodical as you're winding the new springs.
i should remember to wrap an old pant leg or something around the springs next time. when they go, they fling grease all around...
$475 for same day service? 'bout what you'd expect.