A lot of what you're paying for with the "big bucks" vendors is support, maintenance, advice, and depending on your position in the business food chain, some visibility (presenting at their conferences, etc.)

Intangible: what about when your "intelligence" isn't working so intelligently, would you rather be angry with your internal team or some vendor?

In a lot of cases open source BI makes sense, but the cost-benefit analysis should include things other than money.