We start the process to add more peering ports once we see sustained traffic at 40% of port capacity (in PPS or BPS). The process takes a while depending on who you're dealing with, and before you know it, you're up to 80% right before that new port comes online.
Marginal capacity changes are pretty cheap. There's no technical reason to treat network links as constrained resources. So as a network provider, you only create constraint to make money.
Think of it as food & drink in a movie theatre. A bag of popcorn at the Target snack counter is $1.50. A bag of popcorn at the movies is $9. Same product, both high-margin scenarios -- the difference is you're captive to the movie theatre.