What CDN bandwidth to choose?

I’m currently trying to improve the performance of my company’s site. Installing the W3 Total Cache plugin helped a lot but our score is still hovering around 50 in PageSpeed so I was researching CDNs. We’re trying to keep costs down, but I don’t want to invest in something that barely has an effect either, so I was wondering what’s a good amount of bandwidth for a CDN to provide for a pretty basic website with a couple bells and whistles?