I don't sew clothes. I used to, but everything at the store (compared to the cost of fabric) is much cheaper. I will still make some costumes, however. Depending on the quality/price of a bought one on-line. For cosplay/sci-fi conventions you really need a fabulous outfit.
My daughter has been making Assassin's Creed Connor's outfit for several weeks. The con is the 6th. Now, this has cost her a few hundred to make but it's unbelievable. And she's only 16 and never had a sewing class! I just taught her a few basic things and she picked it up from there.