$30 for a 150 foot roll of chicken wire. 

You can do it yourself and it is very simple if you buy this.  Just cut it to the appropriate length and crimp/bend over the edges for strength and safety.  If you have more than one or two, or if you have  done it before, you should buy a whole roll.

For aspen in particular the diameter is fairly consistent--it doesn't have to be much larger at the bottom--and this works great.

The Think Like A Beast Tree Protector is an open source product.

For larger trees 2" chicken wire/poultry netting works perfectly.  It is light and inexpensive.