For hammock use on a budget, I'd suggest looking at the Ray-Way quilt, with Alpine upgrade (for 3 season use). $85 total. (http://www.ray-way.com/php/order-form.php)
Quilt are more versitile than a bag (can be ventilated easier in hot weather, plus accomidate warm clothing underneath easier in cold weather. Bag's are generally a little warmer than quilts when sleeping on the ground since bags have no breeze issues (quilt users can just add a bivy to eliminate drafts though). In a hammock, there is no problem with drafts blowing under the edges of the quilt.
Later, you could make a 2nd one to use as an underquilt for the colder shoulder seasons when you lose a lot of heat from the bottom of the hammock. Much warmer than a sleeping bag since you aren't laying on the insulation and compressing it.
The synthetic quilts don't pack as small as down, but they are easy to make yourself ! You can also buy materials at Thru-hiker.com or OWFINC.com if you don't need a kit / pattern.