It's too early to say that it will cost you that much. I've had sites that have tanked in early days of update and came back better at the end. So far, I don't remember 'losing' a site permanently to any update, other than the Panda, but that was the wild west back then. Good days, when a few thousand russian spam links would get you at #1...

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As for the second part. I don't think that's how you build a working long-term relationship. Are you going to change SEO guys/agencies every 3-4 months or every week, depending on how small updates affect you on day1?
It's not an excuse, it's an explanation of the situation as it is. Whatever you do, you don't get away from an update and nothing is settled before it's over. If you know SEO guys that are constantly 100% immune to updates and rank sites in top 1-3 in competitive niches, please let me know so I can talk to them and learn something

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It sucks, but that's how things are with Google. When you helped a site get thousands if not hundreds of thousands and you showed results before, you're an authority. You've proven yourself, they will trust you. It's all about being long-term, I don't do 1-2m SEO "gigs" and I don't work with clients who want that.