Is it expensive (for bandwidth) too run, say, 30+ rss feeds? I want to let users add RSS feeds so other users can see them too, but I have no idea how much that'll cost me.
It very much depends on how often the feeds are actually read, if you provide any caching mechanisms and so on... Its very hard to put a number on it with the information you are providing.
Just to get you started, I'd do an estimation, such as:
Bandwidth usage per hour = Avg. size per feed entry × Items per Feed ×
Total number of feeds on site × Calls per hour