How much would a 0.41ctw blue diamond be worth?

If the stone is a natural blue diamond, it could be worth millions. However, if it is an irradiated stone, coloured blue by some irradiation process, then the stone is worth significantly less than it would be if it were not a 'blue' diamond.

You can buy a .41 carat diamond today at Blue Nile for less than US$500.

If .41 is a carat total weight, then the value is much less, because the stones are all smaller than .41 carats that make up this total.