The gauges as they are, show how much of your current plan you've used. This makes it look like one is using a lot of the service if you're approaching the upper limit of the smallest plan. A 90% reading on a cost gauge can create a small amount of stress. Rather then that, I recommend using a gauge which spans the entire range of plan levels with marks for each increment. People using only a little bit of data will see a gauge showing a small amount rather than a large amount.
There is a loss of precision by using the large span but, I think you would sell more service due to the lower psychological barrier. Users would no longer be faced with a gauge saying that they've almost maxed out (their $3 plan). and get nervous.