There's an added complication in the example you give, which is licensing, and that the manufacturer fought hard for years to prevent people prescribing avastin instead of lucentis.

They even restricted the sale of avastin for eye use.

It's not surprising doctors didn't prescribe the cheaper med in that situation.

http://www.allaboutvision.com/conditions/lucentis-vs-avastin...

http://www.bbc.co.uk/news/health-30138097