The problem is that it's such a huge jump from $0 to $10k. That's the part that feels like a "gotcha". It's also just a huge number, even for a company with funding.
Google has the same issue with the Google Apps for your domain. The basic version is free (except for domain name registration fees) for 50 users with similar storage/features as gmail. But the business version is $50/user/year. The costs can very easily escalate into thousands.
I agree... there is no middle ground.
I would argue that if your company has 50 people, you should already be budgeting for infrastructure, and the sudden jump in costs shouldn't take you by surprise. And $50/user/yr is still a hard-to-beat price if you're making good use of the package.
what are the alternatives to google apps? i think a microsoft solution would cost more.