If you close $3m on a $10m post-money valuation, you're signing up to build a $100m company (see http://www.angelblog.net/VC_Mandatory_Moonshot_The_Unwritten... ). If things aren't going great (i.e. growth has plateaued), you MIGHT be able to sell for $50m.

This is especially true if you take money from a BIG venture fund (who has $500m under management and needs to see BIG exits to move the needle).

Looking at the chart here ( http://redeye.firstround.com/2007/07/the-unintention.html ), the vast majority of exits are under $100m... Which means that committing yourself to be north of that means a long road and a lot of risk.