This is my strategy as well. I definitely have surges of usage.

Except for one catastrophic binge where I accidentally left Opus on for a whole binge (KILL ME!!!), I use around $150/month. I like having the spigot off when I am not working.

Would the $100/month plan plus API for overflow come out ahead? Certainly on some months. Over the year, I don't know. I'll let you know.