According to this source, the LES system weighed 8,000 pounds. It was jettisoned about a minute after the second stage ignited, about 3 minutes into the flight. Max Q is typically around Mach 1. I don't know how fast the Saturn V was flying at 3 minutes into the flight, but it was considerably over Mach 1, probably more like Mach 5-8.

The cost in terms of payload to orbit for the LES was considerably over 300 pounds, probably closer to 3,000 pounds. Still, the cost of losing another crew (after Apollo 1) would've been considerably higher. Quite likely, the whole Apollo program would've been cancelled or at least severely cut back. It's even possible that NASA itself might've been damaged beyond repair (I know, that seldom happens for government bureaucracies no matter how bad they screw up). Seen in those terms, the payload cost of the LES was a reasonable tradeoff.