Currently most people are paying the $25 flat rate; for this we bump the per-backend memory limit to 2GB and give them a soft limit of 20 concurrent backends, up to 24h duration for those backends, and enable eager image pushing for faster start times. The pricing will become more sophisticated over time as we deal with a wider variety of usage patterns.
The bring-your-own-compute model is a bit clearer because it's simpler for us to model from the side of our own costs. That has a $25/month base fee plus $10 per “server month” of compute connected to the control plane. For bring-your-own-compute there is no cost or limit to the backends that run, since they run on our customer's own hardware.