I've bought a few expansion sets now that they've got an attractive discount that's hard to resist. But the dynamic price behavior is a bit odd. Two observations:
1. Today, on the product page, I had a price of $147.13 for one expansion set. When I clicked the button to add it to my cart, the price shown was $148.52. Not a huge deal, money wise, but an interesting glitch. (A saleperson fixed it for me, anyway.) By the way, before I started the purchasing process, I refreshed my page. I probably even cleared my cache (ctrl+F5), but I cannot affirm that beyond all doubt.
2. Having safely purchased that wonderful pile of resources, I checked the prices on other expansion sets. Some prices went up (usually not by very much); some prices went down (which is what I would expect if the two sets shared resources I did not previously own); and some, of course, stayed the same. (I had recorded the prices of all the XL sets all along my decision making journey.) I cannot understand this behavior, but it suggests that the dynamic pricing algorithm is not as systematically applied as one might expect. This is especially true if there are no shared resources between the expansion set I bought earlier and the others I was looking at afterwards.
Lew