While merely registering for the GMAT costs $250, there's no reason to spend half of your bank account on GMAT prep materials. There are plenty of free resources available to GMAT test-takers.
In no particular order, here are the 21 best free GMAT prep resources:
Official GMAT Software- 90 free questions with explanations, plus two full-length free practice tests.
Beat the GMAT Forums-Extensive GMAT resources and discussion forums. The Beat the GMAT site is an especially good source for practice questions.