Beaded trim is any trim with beads sewn on to it, obviously – but don’t confuse it with beaded fringe. Take a gander at the illustrations to see the difference.
Beaded trims are usually fancy and most suitable for formal wear, Victorian costuming, and anything where the character is trying to say “Not only am I rich, I’m rich enough to pay people to spend hours sewing all these beads on to my clothes.”
Even today, beaded trims are labor-intensive and so they aren’t very cheap. An economical beaded trim will be around ten dollars a yard – you can find cheaper ones but, in my experience, they LOOK cheap. Fancier varieties start at closer to twenty dollars a yard and go up like a kite from there.
While you can’t beat beaded trim for giving your costume a luxurious appearance, balance it with the rest of your project. If you’ve made your Victorian walking dress from $6 a yard taffeta, does it make much sense to spend $30 a yard on a beaded trim to go with it? Contraiwise, if you made that same dress from luscious silk satin, do you really want to go cheap with plastic-y looking $5/yard trim?
Beaded trim should never encounter your washing machine, nor your dryer. If your costume is embellished with beaded trim, hand it to a trusted dry cleaner, or hand-wash it. Some sewists will unpick delicate trim from a garment before cleaning it, and then re-attach it afterwards. It takes a lot of time, but if it’s that or risk having your trim melt at the dry-cleaners, it’s not so bad.