Let's assume it costs $5/month per instance of Office 2000-Whatever. It's really more than that with the MS ASP licensing, but let's use $5 for round number sake. Let's also assume they have 500K users using Office, also what I would consider a conservative number.
That's $2.5 Million/month in licensing costs alone. That pays for quite a bit of developer time.
The real question is, could Office be cloned for less than the user cost of Office? It almost certainly can't be cloned for less than Microsoft's cost to make Office.
That's $2.5 Million/month in licensing costs alone. That pays for quite a bit of developer time.