Pricing errors causing to pay more for less

I normally buy laminated pine shelving from Timber City Roodepoort and the most common size I always buy is 455x2400 mm. I came to realize a long time ago that the price on your system for this size is wrong. According to sales man this piece costs over R700 however a bigger size like 610x2400 costs around R350. Clearly there is a problem on the system when it comes to the pricing on the size I need and everytime I buy there a different salesman charges a different price. There is absolutely no way in budgeting for a purchase since I never know what the next salesman is gonna charge. This is totally unprofessional. I have complained about this matter at the branch several times but nobody ever seems to take this matter seriously. If I was to be an uninformed customer Timber City would charge me more than double the fair price and get away with it. Why can't this matter simply be fixed so that every sales person can give me a fair price that is in line with what you charge for other sizes?