I've modified this sentence, I hope it's clearer now:

They had already received a proposal - expensive but, when compared to similar offers made to other organizations, apparently reasonable — for a managed service hosted by an external provider and based on an open source mail stack. The company offered a managed version with its own proprietary additions and enterprise support.

The catch? While such pricing had become almost "normal" in the market, it was still wildly inflated considering what was actually being delivered. Agency A already had solid infrastructure - reputable IP classes, redundant datacenters, everything running smoothly. We had built and maintained that environment for years, and it was still performing perfectly.

PS: thank you for your suggestion!