automatic monthly payment until end

Hi,

please help me with the right formula giving same payment every # of days.
Example:

Loan: $ 1000
payment: $100
Period: 30 days.
first payment: 1/1/10
last payment: 1/12/10

Billbook is like the calendar:

Column A column B
1-Jan-10
2-Jan-10
...
1-Feb-10 $100
...
1-Mar-10 $100
...
column B above ($100 on 1st of each month), what is the formula giving me the payment?

see dummy file attached as example.
thank you very much.

Carlo