Honestly...it shouldn't be about the money.
It should be about finding them a great home, where they will be taken care of properly and given a lifetime of love and attention.
I paid $400 to take in a breeding pair(the "Breeder" said they were not pregnant but the Momma was pregnant, and they had 2 joeys come out of pouch 11 days after I brought them home)included their cage and all supplies.(These were a re-home situation...the are the parents of my first twins and I was not going to breed them so I took them in knowing Daddy was getting neutered right away, not expecting any joeys for sure.)
I know you spent a lot of money on their cage and supplies, but look at it like it is an investment in their future...that the new owner(s)can provide for them.
I think $400 for everything including the gliders is reasonable because it opens it up without breaking someones bank.
Maybe put a higher price up front but state it is negotiable to the right family, and be willing to cut the price if you find someone who is willing to take them, and take care of them how you want them to be cared for.