There are no fees for executing trades. However, fees will start being charged on March 11th, 2020. The standard rate will be 0.15% for Takers and 0.00% for Makers, with larger Taker fees for small orders. Read more here: https://medium.com/dydxderivatives/the-dydx-business-model-6689aeb1cbde
Borrowing rates float depending on supply and demand and can be found on the Markets page.
5% of all interest payments from borrowers are set aside to fund an insurance pool for the protocol. The rest of the interest payments are forwarded to lenders (prorated by the amount each lender is lending). The rate showed to lenders on the UI is the rate lenders receive; the 5% is already accounted for.
Liquidating a position requires a trade whereby the collateral of the position is sold to repay the borrowed funds. This trade carries a 5% price spread fee.
Expiring a position requires a trade whereby the collateral of the position is used to repay the borrowed funds. This trade carries a 1% price spread fee.