Do your bridesmaids wear robes or necklaces? Decent robes are kind of expensive. Instead of wasting $15 on a robe that they likely won't reuse, I would go with something specific to their tastes. A gift card to their favorite store or restaurant, Starbucks gift card, a bottle of their favorite wine or liquor.
I think a gift card and a card is great. Try to make it to somewhere they enjoy or use often.
I would skip the robe & necklace. I can't speak for everyone, but I only wear nice robes & nice jewelry. My skin reacts to cheap jewelry and I hate those cheap silk/satin thing robes that are typical for getting ready. I'd get them a gift card & thank you note, or something they will use. The better gifts I've been given are fuzzy cute socks, water bottles, coffee mugs, bottle of wine, etc.
I seriously wouldn't waste your money on that. If that's what you can afford I think you'd be better off getting them all $25 (or whatever you'd pay) gift cards to Starbucks or Target or something. Then write them a nice thank you note.
I've been a BM a bunch of times and it would be sooooo much nicer to get something like that. I would actually use it instead of keeping it for awhile because i feel like i should and then giving it goodwill.