Quote from maglia rosa:
The daily price range (high/low) in an option is of no significance at all. It says nothing. The market in the option moves around, whether there is a trade or not, so the low print might not be the intraday low in the option.
A useful proxy to see whether a bid/ask spread is good: take the difference of bid and ask and divide by the option's vega. This gives you a measure of how many vol points wide the market is. When you compare the spreads on options with different underlyings, you have to consider each option's vega. A 0.50 delta option in the OEX will have considerably more vega than a 0.50 delta option in the DIA, and there's even less vega in a 0.50 delta QQQ option.
You are absolutely right that "the low print might not be the intraday low in the option." Since my holding period is several weeks, I don't pay too much attention to option's daily high and low anyway. It's very hard to day trade options.
An option's vega is proportional to its underlying. So DIA's option will be about three times of QQQ's Vega. I have been using (ask-bid)/midpoint of bid and ask as one criteria to select which options to trade. The order is OEX, DIA and QQQ (best to worst). Using your formula (ask-bid)/vega, with the bigger Vega of DIA as a compensator, the difference is even more significant. This measure makes a lot more sense. Thank you!