Scouts and light frigates cost no crystal probably because low end SC units required no gas
yes... because Sins is a total rip off of Star Craft.... Fail!
Its crystal because crystal is what is used in advanced electronics and technology. To research things you need lots of crystal (because experimentation is just a fancy way of saying educated trial and error), whereas to build you need less, but you still need it so it can be incorporated into the design, (like testing silicon chips by using silicon then selling the completed silicon chips without silicon...). Crystal also has structures and formations within itself, its like a natural hard drive, if you can map the inside of the crystal, you can then access and write data directly to and from the material of the shard.
by the way, this is total science fiction and in no way, shape or form factual.
Why Crystal? because it sounds Science Fiction-y =P