well, if stores can rip us off for expensive tank decor that isn't great for fish, I think you should be able to sell smaller ones for at LEAST $5 and larger up to $15. Take into account how much you spend for the glue, the marbles, and how much time it takes you. Plan for about a $1 or 2 profit for each hour or half hour, and add a $1 to the amount you spent for supplies. So say you spend $3 on a pack of 30 marbles, and use 20 in a design. Each marble is worth .10 cents, so that means $2 in expense for the marbles. say the glue is $4 for a tube, and use use a 1/4 of it. Thats another $1 expense for the supplies. It costs, therefore, $3 to make a design that contains 20 marbles. Say it takes you an hour to make it, not including drying time, so thats about $1. In order to make a profit, add another dollar.
So, $5 for a 20 marble design :D :D
. . .Sorry . . . I had too much fun with that . . .