I have a Cannondale R800 for road and a Felt F1X for a cyclocross. Both are standard double drive trains. In comparison, the road bike is quicker, lighter, easier for climbing, more fun overall. The Felt is tougher, fits wider tires--so better traction in the rain, can carry a rear rack and load, and can take the pounding on city streets with less attention to ruts etc., and rides higher which I like.
As fun as n+1 is, I've found that--belatedly obvious--each add'l bike takes time, money and maintenance after the fact to keep in top working order. That's my limiting factor, not the initial cost of the bike itself.