Some of these kits come with free shipping, it can make them hard to beat price wise. You also need to understand a bit about solar to match up the components. Kits make it so you don't need a degree in solar to play with 100w.
craiglist and ebay are wonderful once you know know what you are looking at. My 230w portable was $200. It only took a year to figure it all out.
I would think mounted systems are more popular, then again the affordable portable kits haven't been around long either. They sold so fast that solarblvd couldn't keep them on the shelf. What really matters is to look at the advantages and disadvantages of each and decide what meets your needs the best.