Unless you have heavy experience hosting and tuning prometheus, its not easy to be that cost efficient. It has a tendency to OOM crash on heavy queries if enough memory isnt provided, and provisioning huge memory for occasional queries becomes expensive quickly. Not to mention backups and replicas.
The actual mileage may vary here - we found this to be true for our early customers. Our pricing is simple, and transparent - https://oodle.ai/pricing. Based on our conversations, we consistently hear this is very cost-efficient. Pls let us know if you feel otherwise - but one can can easily input the #active time series / hour to get an estimate from Oodle.