All of the sites are accurate with respect to the dealer costs that they show for packages and options as far as they go. However, you can see that some sites are missing some options and if you try using the sites they may not have the necessary programming for prerequisites to certain choices, or for options BMW allows together but the site doesn't, or for options that BMW doesn't allow together but the site does. Also, these sites do not consistently show training and advertising co-op fees which are real costs per car that the dealership must recover.
To the best of my knowledge, the BMW holdback is 2% of MSRP. I have found thie issue of holdback to be irrelevant because none of the many metro NY dealers from whom I inquired would quote a markdown from published dealer invoice, which would result in sacrificing at least training and advertising costs and then part of the holdback, for an ordered vehicle. The only exception was for an end of MY2011 28i from either physical inventory or already locked-in production, none of which were suitable because I needed three 35i, all with different configurations. Production slots are assigned as either 28i or 35i. Dealers typically order a generic configuration for every production slot that they have been allocated; I suspect that every slot has a drop dead date to use it or lose it. The dealer must pay at order time. When the dealer gets a customer order, the production order gets a customer name and, typically, a configuration change and becomes a Priority 1, thereby enabling Priority 1 options. If no customer order materializes then the vehicle becomes destined for physical inventory on the lot.