That would be the cost if someone was to buy a stock gun and a bunch of upgrade parts then do the work themselves. If they had to hire someone to do the mods it would be more.

However, the E7 demonstrates that you can purchase a weapon with superior and it doesn't add 5k to the cost. At only 1200 credits the E7 is only 800 more than a regular blaster pistol.

Yeah. I guess 5k is the difference between starting with a weapon with superior design and construction vs. a weapon where you have to fix 1001 quirks and inefficiencies in order to make it superior.