What is the base cancer rate for an arbitrary carcinogen?
To answer this question in its entirety we have to split it into two questions:


  What are the underlying mechanisms of carcinogenity?


One of the main mechanism behind carcinogenity is the mutagenity of the cancerogens, i.e. the ability to cause mutations, that are abberations of the cell DNA leading to uncontrolled proliferation. This classical paper investigates the relation between cancerogenity and mutagenity.

One should mention here that there are many types of mutations possible, mutations are not equally dangerous for cells and some mutations can be successfully repaired using the intact strain.

Therefore the following parameters of the source substances need to be measured to estimate the cancerogenity:


Substance concentrations or absolute amounts (both being indirect measures of their chemical activities, the lower concentration is needed for causing cancer the higher is the chemical activity and vise versa).
Substance radioactivity (for the mutagenesis due to radiation).



  How can we measure the carcinogenity of different substances?


The most general approach here is to introduce certain amount of cancerogen into the animal body or to the cultured cell and to see the effect. The effect is calculated as the percentage of cells that undergo the transformation from normal into cancer cells. Two metrics are available here:


  DT = Tumorigenic Dose (the amount of substance causing certain
  percentage of cancer in treated animals, all treated animals are taken
  for 100%)
  CT = Tumorigenic Concentration (same, but adjusted for
  concentration and used in cell cultures).


(They are written CT and DT because in science people tend to used Latin abbreviations where the adjective actually follows the noune).

The common metrics are DT5/CT5 (5% cells/animals get cancer) and DT50/CT50 (50% of the animals). Those are similar to other common metrics, the most common is LC50/LD50 -- lethal dose for 50% of the animals/cells.



Unfortunately I couldn't not find any pre-compiled list with most known cancerogens and their TD/TC values. These seem to be interesting primarily for scientists. But going back to your question: you are absolutely right: some cancerogens are much more potent in causing cancer than the others!