Usually big brands buy parts in huge numbers, including stocks for future repairs. So they have a firm number of stock for each production run at a certain price. When the production run ends, if there is an escalation in part prices, then prices of kit are increased. Though this offsets short price escalations, this leads to higher pricing as the interest rates on all hoarded parts have to be factored into the final pricing. Plus, they will be left with huge inventories if the sales dont match their market research.
However, our local manufacturer's are mostly building on a order basis, with no stock piles of inventories. So they are very exposed to volatility in part prices. Hence, the need to keep revising prices every few days as their suppliers keep updating on the revised part prices. So their prices are lower, which is what we all want eventually, but fluctuating. Hence, they cannot post firm prices on their sites I suppose.