Why is it expensive to enable DAI lending on Compound/DyDx Exchange?

I’m new and trying to figure the technical reasons of what happens when an exchange like DyDX or Compound Finance is authorized to enable a stable coin stored on a wallet and start lending? It seems to be expensive as I see a prompt on the wallet app asking me to pay fees which is in the order of 20$-70$ depending on the gas price at the moment.

Also, should I pay a similar fee if I’ve to enable the 2nd stable coin? Or is it a one-time fee per wallet? Are there any additional fees involved when I try to lend after enabling, let's say DAI on DyDX? Where can I learn more about them apart from reading lengthy & complex white papers (sorry, they are too much for me at this stage)?

One area I could imagine is – Compound / DyDx is trying to create a contract on the blockchain and I should pay money for storing the contract. Is my understanding correct?