Mega Burg never developed or invented a unified, formal currency. Most trade is done through bartering, and every item has some distinct value. However, one informal form of currency accepted by everybody is food. The more exotic or long-lasting the food the better.
Since there is no formal exchange rate for food among different Seekers one had to learn to haggle or find those rats who wanted something they had.
This said, some seekers, and groups, invented their own forms of currency to make the exchange for certain goods easier. These tokens, „fingers” or ballots came in different shapes and sizes and could usually only be exchanged with a specific seeker or groups. Because of this, these forms of currency were only usually as stable or useful as the seeker or group they came from.
The colony authorities recognize only food as currency (though in a few cases other forms of goods were also accepted or paid out with), and most rats are paid in food for services or work performed for the colony.
The best denominator for the value of food is how readily available it is. The rarer the type of food the more valuable it is, and if it’s highly perishable it’s value could be higher (although for practical reasons it must be eaten instead of being traded).