Regarding En Primeur and weather reports, Gavin Quinney has an extensive weather report on Bordeaux yearly and you can chart the weather versus price ratings. Even before the wine critics land on Bordeaux for the yearly EP tastings, other wine lovers are able to access to these information and gauge the wine prices before a single point is awarded.
On the Banks of Bordeaux,
The left bank is suited for Cabernet Sauvignon as it's more gravel based, hence retaining heat to allow for faster ripening for the difficult grape such as Cabernet Sauvginon. On the right of the Garonne river, there's more clay, which retains water and doesn't have as retains head as well as Gravel, hence early ripening varieties such as Merlot would do well here.
Who sets the price? The insider question would be: Who makes the most money - the wine marker/chateau, broker , negociant, importer or retailer? The La Place de Bordeaux, the centuries old mechanism set in place in Bordeaux has the entire chain of the above, with the Courtier (or broker), setting the pace in the early times (they're instrumental to set up the 1855 classification). In modern times, the lines are blurred, as retailers or even end consumers are buying directly from the wine maker. More information on La Place de Bordeaux is detailed by Wendy Narby here.