I've got a few questions on the optional regimental starting gear, nothing major. Hopefully some of it was resolved in questions in the beta or will be expanded upon in Hammer of the Emporer.

Firstly, with main weapon upgrades, as there doesn't seem to be a cost saving in buying a weapon as a replacement for a lighter main weapon was it not intended to be able to buy weapons with the any item of a given availablility option.

And not really bothered with extremes, rather than stocking up on several basic weapons or even a heavy weapon per man. Say you end up with a las pistol as your mainweapon (say because of the Warrior weapons doctrine), If you were still interested in keeping the pistol, would you need to replace the pistol with a lascarbine and then also buy a las-pistol as a sidearm.

Secondly, what's the cost of a single item per squad? Say a Vox caster or a few demo charges. Is the same as getting one per character? The only example of a per squad item is 6 screamers which I imagine is about the same as a one per character in many cases.

Actually anything bought through the "other items" section is at GM approval, so it comes down to your warrior weapons regiment trying to justify ALSO having high powered weapons to make up for the flaw they opted for… GM will likely not approve.
Mainly i see the other items section as a way to do funky stuff like get extra flavour into a regiment, rather than min max the weapons and armour.

For example i created a siege regiment earlier that get the usual screamer box per squad, micro beads etc but then using the 'other items' bit i added in accordion wire, man traps and a melee attachment for their combat shotguns. These guys dig in hard and wait for the enemy to come to them, then turn them into red chunks.

I'm not in favor of min-maxing, but I think the way they did it is pretty silly. Some of the things are really expensive while others are non-sensical. Like the bullpup lasgun.

At GM discretion, I let them remove or add a weapon to their Standard Regimental Kit or "Specialty Kit" (the equipment all Specialties start with) if I thought it was sensible and made sense. So an item of Common quality would typically be 5 points, whether they're removing it or adding it. Weapons would come with 4 Clips, single shot weapons came with 10, and expendable weapons like grenades came in boxes of 20.