Basically an idea that won't cost the server much or any ram, the idea with spawners which don't spawn cost ram
But, the idea with sponge block is interesting, say, every person in the server is given 50 sponge blocks. They can go to the server stock market ( a hypothetical place right now ) where they can store them in a specific sign with a stock name.The stock's price will change every 15 or 30 min. or so. This will allow the user to withdraw ( when he wants ) a specific amount per day (real day). Suppose he/she is allowed to withdraw only 10 sponges per day and his profit or loss may be like this
'' Initially when Debu stored 50 of his sponges in the MinecraftMinersGuild stock at the price of 5 sponges per stock ''
'' Later , Debu withdraws those sponges at the price of 4 sponges per stock ''
So he lost 1 sponge per stock. That means a total of 10 sponges were lost
Now what with the sponge?
It should be bough by the ingame money ( or tokens as davo said is a good idea ) and sold for ingame money.
suppose a sponge is bought at $400 and it's selling price is $250