When you practice firefighting, do you use different, cheaper life support gear than you would use in a working situation?
If you're going to practice for tree climbing, then use tree climbing rope. There are many good, inexpensive 16 strand arbo ropes available.
A couple days ago some of the guys here were talking about a guy on eBay that sells short ropes quite cheap. 60'-80' would be more than enough to get started.