Which is more expensive, servers or developers? Which is more expensive, server or opportunity cost?

In a lot of cases, performance of the server isn't the determining variable in the overall cost calculation.

In addition, the relationships between those variables likely changes over time.

I'm guessing that for most startups, HR expenses and opportunity costs are pretty important relative to actual server performance.

Obviously, if the point of your startup is some kind of high-performance computing thing, the equation is tipped more towards server performance. But, you probably aren't using off-the-shelf frameworks in that case, either.