It depends a lot on what type of hitters you have... but the for the money the bat with the best pop I have used that is widely available is the Demarini Rayzr, I have used many of the lower end 50-130$ bats and this one at 80-90$ish was a lot better than any of them imo, and great for softer balls. Obviously you want to try to avoid using it(or any bat you want to keep for a while) in cold weather. The Demarini Ultimate Weapon is supposed to be a very good cheaper bat, but it has durability issues and might not even last a season as a community bat. Just make sure the bat you get is the best "circumstances" for your league... Many leagues ban additional bats as well as non-asa leagues, specifically Coed-leagues. Other types of banned bats include all double wall, and or all composite bats in addition to the ASA spec depending on your league. I did a ton of research and read about a ton of bats, but was unwilling to pony up the cash for a nice bat that would get destroyed by my team. You also need to find out which compression you use, as most bats are designed to hit a certain compression better, and using a bat designed for softer balls on harder balls will cause them to dent much quicker.
In general in my experience Co-ed leagues will often use the softer .44/.47Cor with the .375 compression to limit ball flight.
All in all I found the entire process to be extremely frustrating and eventually went with something cheaper, which I have been very happy with and most of my team uses over a Demarini White steel, or a few others.
I would recommend _not_ getting a used bat through Ebay. When you buy it new you get a very easy to use warranty with nearly every bat company so when its dented inside a year you can just send it back with little questions and receive a new one. It can be hard to tell from pictures if its warrantiable or not(the ones that aren't have small writing you often can't see in pictures). I guess if you can ensure that it is in fact a NEW one and not a replacement one it might be ok. You also need the original receipt to do this.
Check out my blog at http://www.rotopulse.com for fantasy baseball analysis, injury reports, commissioner info and more.