Crochet headbands are interchangable themselves. No need to alter them. Just clip a bow on through one of the holes.
I sell mine for $1.00 each without anything on them. However..I dont' sell too many of them 'cause they aren't popular around here. NOTHING crocheted is.
If a bow or flower is attached, it all depends on the size. With a regular 4" flower, it would be about $5 or $6 here. A little more for a reg. bow. Anything over the top or jumbo, of course you'd add more. I have sold some crochet headbands with Jumbo peonies on them as photo props and they were $12 each.