While Mega Burg might differ greatly from human civilization present right above it the rats have developed an economy of their own, with their own rules, regulations and laws. There are, of course, many similarities between the rats and humans, but do not expect to see any minted coins here!
In general, rats in Mega Burg trade with one another through barter, food in its many forms is considered primary currency. While individual seekers (merchants) and certain groups of rats tried to introduce their own currency forms (such as coins, tokens or ballots) they were only worth as much as the group issuing them, and thus not very stable.
Seekers are akin to explorers, looking for things that they know could be traded away in the main colony for other worthwhile goods. Meanwhile makers (craftsmen or craftsrats) try to manufacture their own goods from recycles or improvised materials. Both groups are often faced with their own problems. Seekers tend to face problems in the form of taxation, banditry and thievery. Meanwhile makers try to find more efficient and effective refinement methods, which are largely restricted due to a limited understanding, as of yet, of how humans came to create certain objects.
Taxes are not universally applied to all rats, seekers being the main group having to suffer through their burden.
Trade between smaller colonies and the main colony does exist and most „caravans” (if they can even be called such) travel through the sewers, as to avoid unwated attention from the humans.