If you got here like I did, note that these do not grant extra bargain attempts like the other tiers of Trader's Clothes.

For +1 you need 9-10 Increase Enhancement rate

I found this to be ok,

+1 costume i will smash at +0 get about 30-40 of them
+2 costume i will smash at +17 push it to +20
+3 costume i will smash at 26-30
+4 costume i will smash at 40-44
+5 costume i will smash at 60-70
Someone posted this on the BDO forums and it seems pretty good to follow.

But i actually do this...
5FS +1, if it goes to 10FS, go for plus +2, if it goes to +15 Buy 20 Advice for Valks and save it.
20FS for +3.
But i havent gotten any further than that as of yet.