How to use agents via API cheaply?

I’d like to code with an AI agent via a BYOK open source client. I want to be able to inspect everything, check the system prompt, etc. Also, I’d like to avoid vendor lock in.

Is anyone here using a setup like this?

Does this get close in pricing and efficiency to the subsidized plans offered by model providers (like Claude Pro, etc)?

In particular Deepseek and Kimi models are being praised as both efficient and cheap; I’m curious if anyone built a coding setup around them.