I've never experienced much difference between student grade and what's labeled as professional grade oil paints. One reason is that I work with Lukas paints and there's just not much difference in the line. Make of that what you will - that they are uniformly excellent or they are uniformly student grade. But I've used labeled professional grades, including paint from small, high-quality producers, and, again, just don't see the difference, other than a tube or two of TV-painter paints. Those last were pretty disappointing.
A second reason is that there are relatively few pigment producers world wide. All of them are primarily producing pigments for automotive, which calls for incredibly cheap, highly consistent, light-stable and environment-stable colors. Old Holland and Blockx likely source many of the same pigments as those used in Winton and Lukas Berlin.
A fair number of these pigments are, gram for gram, pound for pound, cheaper than even the cheapest linseed oil -- and a good percentage of them are cheaper than fillers. There's less and less incentive to cut product by adding oil or fillers in cases like that.
This, of course, doesn't apply to true mineral or clay colors (there are very few left being mined and milled), such as true ochres or (most assuredly) to lapis...