330 plus or minus 5 is the best you can get on the market. However if someone purchased them prior to the limit being dropped from 350 to 330 you could get 345 plus or minus 5. The other way to get them that high would be to make a unit then upgrade to the level you want (450 if not a mobile). However at that point it is no longer cost effective to sell.
If the item is profitable enough (and for a weapon that is hard), you can create an Enterprise controlled Public Corporation (upgraded to 250), output 340 - 347 using 210 Supplies, otherwise you wind up with 305 to 315 using the same supplies, also Enterprise controlled (upgraded to 225).
So far, I have seen Precision Bombs, Cruise Missiles Ship Based, Land Based Cruise Missiles, Airforce Maintenance Units, Defense Fortifications, and Military Bases Maintenance Units successfully pull that off.
Strategic Airports and Strategic Military Bases made the IPO but do not appear to be stable. Most of the rest I have looked at lose to much money to ever IPO.
So if you want something between 300 and347 I may be able to find some surpluses I can sell.