It is possible that you could replace the battery ~$5, there are several guides here on iFixit, and one of them is a stinker, and some of the others are better. Anything that is 2 steps long is not detailed enough for non-experts. Once you replace the battery, you'll want to do the PRAM reset trick one more time.
This may be a long-shot though, even with my battery dead, I was able to get my iMac to boot sometimes, but I've seen Windows PCs stay dark until the battery was replaced... so, this step is cheaper than buying a new Mac!