How much is a driver's side headlight for a 2003 Nissan Sentra XE?

Your Part is 9007. A Sylvania Long Life bulb is $6.99.
You can buy brighter bulbs such as Cool Blue or Halogen Super Bright. Keep in mind the brighter bulbs burn out faster.

http://www.partsamerica.com

That is were I typed your car year and model to find the estimate and the part 9007 for the bulb.

You should be able to find the bulb at a local autostore or Walmart, Target Store.

It depends on what kind of bulb you want to use as a replacement. Just checking Autozone they can run from about $11 for 1 regular bulb to $50 for 2 Ultra Night Vision bulbs.

Here is a link to Autozones list:

http://www.autozone.com/N,15700044//shopping/partTypeResultSet.htm

Don't be an idiot; cost depends on where you buy it and whether you include installation by a certified mechanic.  If you can find a #2 Philips screwdriver and a car parts shop though...and I expect you don't have HID headlights...you can expect to drop $10-45 depending on the quality of the replacement.