kperrin,
In the example of GLD you are using, this might not apply so much, but sometimes a certain month might be judged as "overpriced" because of an event such as an earnings release that is coming. So it is good to know the relative price of what you are buying and what you are selling.
As far as the Etrade calculator, you would do good to figure what the prices should be on your own.
Larry McMillian each week updates options volatility data that you can get here:
http://www.optionstrategist.com/free/analysis/data/index.html
It shows that for GLD, the IV is around 46, while it's HV measured at different times is between 42-47. You could enter the IV along with the days remaining, etc. into a B-S calculator and come up with the "fair" price.
You can find some calculators at
www.888options.com. Look in the menu for "Trading Tools" -> "Pricing Calculators".
I don't know if ETrade does this or not, but optionsXpress, which I use shows the current IV for each actual option in the chain, not just the general IV for the stock. I think that's a nice feature to have.
JJacksET4