FR-4 has a lot of variation from vendor to vendor. If you can be assured that your vendor uses the same supplier of FR-4, it is fairly consistant from batch to batch. It is not the best to use for RF circuits that need good control of both the dielectric constant as well as the lost tangent. However, it IS used for RF circuits in spite of its weaknesses.
I recommend that you do Monte Carlo type of analysis on your design to see what effect these parameters have on your overall design.
Cheap prototypes on Duroid? Never heard of any.