Yup, that is a mitigating factor. And as you say that is highly unlikely considering almost all other items are either same as TP or lower. And GW has consistently described RAI as difference in cost as discounts only. Discount is obviously intentional. I see no sign of the opposite. So I took the liberty to clearly label these as erroneous. Also consider these costs has changed over the years. For example being same in gang list and TP, then later reduced in TP without same reduction in gang list. This causes a problem with calculating rating - which cost to use? Again, considering how the intention is to use gang discount cost, this would be a penalty. While you get a discount for spending credits by finding it at the TP, the rating still uses the higher gang-specific cost. The alternative is to keep track where you bought each individual weapon. And to what end? How does this improve the game in any way? It sounds complicated for no real gain.
And thanks for the tip - next version of Necrodamus will have red font for Genestealer Cult's needlepistol cost of 40.