i did the math on that once and unless u have all the adena u need for the upgrade, it will come down to the price there seiling for right now, which is like around 10.5mil.
A lux weapon is 6.5mil max if you buy all ur crystals from private shops.
the upgrade is roughfly around 2mil AA, which translate to 4mil reg money.
So ur at 10.5mil alrady, so by from private shop.
On the other hand, if you save money on crystals n AA then, u will fall under the 10.5mil mark, so then go for it....