Honestly, mono red burn isn't the worst thing. Boros is definitely much better, but that's because Boros Charm, Lightning Helix, and white sideboard cards are all really great.

In my experience playing various burn strategies, Seal of Fire is only really good if you can recast it. I'd honestly give the edge to Light up the Stage, Flames of the Blood Hand, or Exquisite Firecraft. Other options would include Searing Blaze 3-4 and Grim Lavamancer.

That said, the majority of the cost of any Modern deck is usually the manabase and sideboard cards.