1. No bid , ask=100 2. Bid = 100, no ask 3. No bid , no ask How do brokers calculate PNL or margin for these cases?
Depends on the broker and what they're calculating it for. IB at least used to count the value at zero if there was no bid and start autoliquidating you, but The Algorithm was stupid enough you could put a bid in low enough not to get hit and it would use that to MTM. I think real brokers use last.