I used Seeedstudio a few weeks ago to do 10 smaller PCB's on the 50x50 option.
The price was $13.45 delivered with green mask and 50% E-Test.
Seedstudio charge $10 eaxtra for red/blue/yellow and $20 for black. They also charge extra for 100% E-Test.
They also sent email progress but no photo's. Those boards were back to me in the UK in about two weeks and I was very pleased with the quality.
I'm now designing, where possible, for multiple PCB's onto 50x50 and 100x100 panels. Moving to 200x200 for panelising doesn't seem to have any advantage. It's actually more than 4x more expensive than 4x 100x100mm
From the look of the photo's, and the fact it seems Elecrow and Seedstudio actually use the same manufacturer service, I'm expecting these to be as good quality as my first batch via. Seeedstudio.
I just hope it works! I made some minor errors on my small boards, but not to the point they were unusable. This one has been checked to death before submitting.
Rendering the Eagle designs in 3D, and being able to rotate and check from every angle, is invaluable.