If you mean what price you can get executed at immediately, you will generally pay the offer and sell at the bid. If you are trading with limit orders (always wise, unless you need out in a big hurry), you can, of course, try and do better, and a good floor broker can usually at least split the spread and get you executed somewhere in the middle. The last price traded is irrelvant in all but very liquid markets, as it can be very recent or very old, and as a result, in options, it is generally pretty meaningless.
Jessie