Another consideration would be to compare the buy in of $179.99 vs $319.99 Other than simple forces of the market as @keymomachinementioned, buy-in will likely be the #1 driver in the high price. For comparison, the construction site has reached 2.09x of rrp vs 2.5x rrp for the Store. The supply is similar (10,000 units) and buy-in was same for everyone, so their secondary market performance will most likely be similar as is the case now. One big difference being upfront capital invested (1.78x.) Either way, as long as modular demand remains high, both of these sets will remain strong performers for the foreseeable future.
Now obviously all of that analysis is thrown out the window if Lego decides to produce their own Modular Store or Construction site.