What is the relation between plasmid concentration and mRNA levels?
The amount of transfected plasmid does not correlate at all with the protein expression level. After transfction, usually each cell is going to get and keep only one copy of the plasmid. Once the plasmid is in the cell, it will be replicated and the cell will contain X copies of it, depending on the plasmid copy number. In general, plasmids with low copy numbers correlate with lower protein expression compared to plasmids with high copy number. But there are also exceptions:


  Expression of genes under control of EF1α promoter appears to correlate with plasmid copy number.  In contrast, expression driven by the more traditional CMV immediate early promoter appears less plasmid copy number dose-responsive (ref).