I do agree with Kelsey's concept of wedding accessories to the BMs and MOH dress jewelry to match the day for your wedding. Robes can be one of them too when it comes to the getting ready time to give them. The colour and style can be up to you and something they will wear for other occasions.
Sterling silver is very affordable and is pure silver mixed with usually copper. They should be hypoallergenic but you can always ask your jeweler.
These are examples of sterling silver necklaces with cubic zirconia from Bittersweet priced around $65 each but they currently have a Buy 2 get 1 promotion so if you have 3+ members of the bridal party (you can include the two moms if you want to round the total to multiples of 3), it'd be under $50 per piece.
If you're willing to stay in the $60-70 range per piece, you can find items at Peoples with cooler gemstones that match each girl's personality. Since your wedding is not til next year, you can wait until one of their sales on women-centric holidays like Valentine's Day or Mother's Day though they also have sales on Black Friday and Boxing Day.
But if you prefer to stay under $50, typical jewelry chains have tons of sterling silver necklaces at that price range. This is from Michael Hill (unlike the People's stuff above featuring rubies and sapphires, these are cubic zirconia).
I also really like this fine silver pearl necklace from Birks for $50. This is fairly affordable while still super fancy.
When buying, just make sure it's actual sterling silver and not silver plated. Silver plated usually means it's a plain metal alloy underneath that usually is a mix of nickel and copper.
Are you wanting specifically items that are intended to be worn during the wedding? If not, scrunchies colour coordinated with your wedding colours are popular for each girl. Tumblers with their names on them, or makeup bags/clutches with their names