Okay, i know it's going to cost me to store extra copies of data, but how much exactly? If I have a replication factor of 3, does that mean I need 3 times as much storage?
A blog post about storage requirements
http://blogs.splunk.com/2013/01/31/disk-space-estimator-for-index-replication/
Not necessarily. It depends on your search factor as well. With a replication factor of 3 and a search factor of 2, you're storing just two complete copies of your data. For the third copy, you're storing only the rawdata file, reducing the size of that third copy by a significant amount.
For more details, including examples, see this subtopic on determining your storage requirements: