I would recommend a limit on workflow (+ background API workflows) steps run and records loaded from the database. Each application uses workflow steps and loads records from the database. So you could limit that. Also its something very measurable.
Free - 100 workflow (steps) and 100 records loaded a day
Hobby - $6 /month - 1k workflow (steps) and 1k records loaded a day
Personal - $29/month - 2k workflow (steps) and 2k records loaded a day
Premium - $60/month - 5k workflow (steps) and 5k records loaded a day
Professional - $129/month - 10k workflow (steps) and 10k records loaded a day
Production - $320/month - 25k workflow (steps) and 25k records loaded a day