Is it possible to set up calculations based on day of the month?

hi

i want add a field onto my application form that auto generates a price depending on what day of the month they are joining on?

Say £490 on the 1st and £480 on the 10th.  

I am not sure if it would be possible to set a price based on the day value in a date field.  It does seem possible to pull out only the day value on use it in conditions.  You could do this based on day of the week (Monday Tuesday Wednesday etc) but not based on the day number.