There are at least 4 major varieties of Large Cent from 1803.
Numismedia lists the following approximate retail values for the most common "small date" varieties, as of 05/2009:
Very worn condition - $65
Moderately worn - $200
Slightly worn - $400
Almost no wear - $1500
The rarer "large date" coins can sell for anywhere from $100 to $26,000 so regardless, it's best to have any 1803 cent inspected by a professional dealer or appraiser.