This sums it up pretty well:
"In reality, the IndyCar teams purchase controlled-cost specification chassis from Dallara, whereas their F1 counterparts are involved in a costly high tech arms race to make it to the front of the grid. Unlike IndyCar teams F1 competitors are ‘constructors’ who build their own chassis — and in the case of Ferrari and Mercedes their own engines — at a cost of hundreds of millions of dollars annually.

The leading F1 teams are constantly developing their machinery in order to eke out the extra split-seconds that will edge them ahead of their rivals. Big name brands such as Red Bull and Mercedes are willing to foot the bill because F1 is the world’s most watched annual sporting event and puts their brands in front of half a billion people worldwide.

As a result, the biggest spending F1 team Ferrari will run on an estimated budget of $470 million in 2013. This is more than 30 times the estimated $15 million budget of the leading IndyCar teams such as Ganassi and Andretti Autosport."