The point to note is Honda is trying to decrease its spending by streamlining same exhaust muffler in 100,125 and 125-5speed models..In short it is not an innovation.Each and every bike must have its own design/compatible exhaust..The original CD-100s exhaust is larger in size.
When a product is streamlined in mass production--result=low cost of production
So in short if Honda introduces alloy rims in all the 3 mentioned models..Its expenses would be the same/near to the spoke rims.