Things might have changed but Titoff used to be the cheapest on the market.
https://www.belize-offshore.info/index-en.html
They were barely adding any margin at all. I can't vouch for their quality, though, as I can't remember working with them. In my experience, though, you get what you pay for. Saving a little bit on renewal fees can end up costing much more if the agent is unable or unfit to help with other parts of your business.