how to calculate marginal effect?

Hi there,
Need some help calculating marginal effect.

Variable	Description
sid	Subject ID
age	Age
famsze	Size of the family
educyr	Years of education
totexp	Total medical expenditure
retire	=1 if retired
female	=1 if female
white	=1 if white
hisp	=1 if Hispanic
marry	=1 if married
northe	=1 if North-East area
mwest	=1 if Mid-West area
south	=1 if South area (West is excluded)
phylim	=1 if has functional limitation
actlim	=1 if has activity limitation
msa	=1 if metropolitan statistical area
income	annual household income (in 1000 dollars)
injury	=1 if condition is caused by an accident/injury
priolist	=1 if has medical conditions that are on the priority
totchr	# of chronic problems
suppins	=1 if has supplementary private insurance
hvgg	=1 if health status is excellent, good or very good

My question is 'What is the marginal effect of annual household income (income) on the total medical expenditure?' Given marginal effect is the slope of the regression, I am a bit confused on how to answer this question. Any help would be greatly appreciated.

Thank you!