I have made a costume before, it was a Fallen Angel Flonne outfit, but it wasn't made very well.
For my Haruhi outfit, I bought it off on ebay for $70 which is pretty cheap for cosplay clothes.
In my opinion, making your own costume is more expensive than buying it, because you need to know how much material is needed, so you'll need mock material (which is basically cheaper material, but you need it to test it out). After you try out the mock material, then you need to search for the real material which some parts might be expensive and some not and you need to see which texture would work best.
All in all, handmade costumes are usually better (if you work hard on it), but there's a hard way and an easy way. Easy way being buying the costume.