The Cochrane Collaboration are a well-respected organisation that do meta-analysis reviews of other studies. That is, they systematically examine all of the studies related to an area, discard the ones that are not high-quality (according to a pre-determined system), and pool the results to produce a high-quality result.
So, the fact that there is a Cochrane Review of electric toothbrushes makes the skeptics' job much easier. Someone has done all the hard-lifting. That's the good news.
Source: Deacon SA, Glenny A-M, Deery C, Robinson PG, Heanue M, Walmsley AD, Shaw WC. Different powered toothbrushes for plaque control and gingival health. Cochrane Database of Systematic Reviews 2010, Issue 12. Art. No.: CD004971. DOI: 10.1002/14651858.CD004971.pub2
They concluded that some types of tooth-brush might be a little better than others at removing plaque and gingivitus.
The review included data from 17 trials with 1369 participants. There is evidence from seven trials of up to three months and at unclear/high risk of bias that rotation oscillation brushes reduce plaque (SMD 0.24; 95% CI 95% 0.02, 0.46) and gingivitis (SMD 0.35; 95% CI -0.04, 0.74) more than side to side brushes. Due to the dearth of trials, no other definitive conclusions can be stated regarding the superiority of one mode of powered toothbrush over any other. Only minor and transient side effects were reported.
The authors summarised this as:
This difference is small and it's clinical importance is unclear.
The bad news is that despite the fact they set out to look at other factors ("calculus and stain removal, cost, dependability and adverse effects"), they didn't find a conclusion ("Cost, dependability were not reported.")
While some electric toothbrushes mechanisms might be a little better than others, there isn't much in it (and I can stop looking, because if Cochrane didn't find it, it isn't there on the available evidence in the literature.)
I can't answer whether the expensive ones correlate to the better ones. I can note, informally, that cost, when it comes to consumer goods, may be regional and more related to branding, colour, battery-life, dependability and other factors not related to the toothbrush's efficacy.