At least in the sciences, and in the US, there is also the issue that research professors tend to be on "soft money" -- that is they get a minimal salary from their institution but can increase it (up to a point) by getting grants that they can charge their time to. And they also tend not to be in the tenure track system. That being said, if they get large enough grants, they can make as much if not more than traditional tenure-track professors with defined salaries. But in years where they don't get much grant funding they don't make much at all (I used to be an non-tenure track research professor myself).