Use the search function. It'll give you some good info about that well-discussed topic.
Basically it boils down to this: It is possible to climb trees with rock climbing ropes. Rock ropes were not designed for arboring, so your climbing WILL BE LIMITED.
Good tree climbing ropes are not expensive. You can get 120' of sportline from newtribe for $80.
New Tribe's Sportline
I started on rock ropes. It was different because I am a rock climber and I was just using rope that I already had. It would be a waste to buy rock rope for tree climbing....no matter the price.
love
nick