According to who?
Salt as a filler could be applicable when it comes to VERY heavily salted baits but the vast majority of baits on the market with salt in them the difference in cost is really negligible.
My take on this is the vast majority of people making baits that tell you "Salt is for french fries" and that salt doesn't make a difference etc. don't want to deal with it. It can be problematic figuring out how much to add to do what you want, what type of salt to add and some of the moisture issues that can pop up with salt (must keep it in a cool dry storage as it will absorb moisture).
You will need to really play around with the amount of salt to add to the plastic you use in order to achieve the results you are after. Yes you can make transparent baits just fine that have a noticeable amount of salt in them, Yes you can make baits that will be neutrally buoyant, claws will stand up etc with a noticeable amount of salt in them, Yes you can make stick baits loaded with salt that are still relatively translucent and show flake within the bait and finally Yes you can make a fairly durable bait with salt in it, even a stick bait LOADED with salt can be fairly durable if you choose to use a very fine grain salt. Don't let the nay sayers sway you against using it, try it out and really mess with it. One little tip, use the finest grind you can buy, DO NOT grind it yourself, I found after years of messing with it that grinding it causes it to really cloud up and clump, using a VERY fine grind eliminates 99% of those issues.