It's the time in between that's the most awkward. When the potential savings are there that hiring an engineering team to internalize infrastructure will give a good return (were current pricing to stay), but you're not so big that just threatening to leave will cause the provider to offer you low margin pricing.
All I'd say is don't assume you're getting the best price you can get. Engineers are often terrible negotiators, we'd rather spend months solving a problem than have an awkward conversation. Before you commit to leaving, take that leverage into a conversation with your cloud sales rep.