I've done a few album covers, and it really depends on the amount of time it takes you, the materials, and how well known the band is. More well known, you take advantage of that.
First off, decide for yourself how much money you would regularly charge an hour for any sort of graphical designing. How much do YOU personally think your time is worth. Then, estimate the time it takes you, plus the amount of money for supplies. Then take into account the amount the person is looking to spend.
I always ask "Hhow much are you looking to spend?" first; although its known to be frowned upon in the professional world of graphics, it can easily decide on a more accurate approach to whether or not they've looked into other designers. Usually they'll lowball you if you know the prices in the market.
First above all though, make sure you have a price in mind. Go over your estimated time and supplies on a hourly wage, then shoot the price up another couple bucks.
Personally, I usually give them the best price possible, watch the market, and then do it for them for $25-50 cheaper than the lowest.