Options pricing is per contract, so a four-leg iron-condor costs four times a single call, opened and closed. Exchange fees, the regulatory fee and index licence fees stack on top and are usually invisible until the statement.
For small premium sellers this dominates. A strategy collecting $40 per trade cannot survive $12 of round-trip friction, however good the edge is.
Example: at $0.65 per contract, a ten-lot iron condor costs 40 contracts × $0.65 = $26 to open and the same to close, or $52 round trip plus exchange fees. On $400 of collected credit that is 13% of maximum profit paid to the broker before the market has done anything.
Related: spread-width, penny-increment-program