I would need a web form coded in PHP using REST/cURL to write to and access xml/ json data.
The program should of course have a web interface where each member can enter their personal data (firstname, lastname), and fill in the sum they have payed on a monthly basis. The information should be able to write, update, edit and delete (by using REST).
The payements of each member should be added up to a total sum and also show how much is still missing from the total of each member. Also a nice feature would be if the total of all members payments would be displayed and also here how much is missing from the total budget. As far as I understand, it requires that the total values has to be inserted somewhere which I can do myself in the php-code so there isn't a need for an admin-interface.
The reason for PHP is that I know this enough to be able to modify the code myself one day if necessary. Xml/ json because I don't want to install SQL databases for this.
Bonus: It would be a nice add-on if the data could be presented as a graph (Google Graph or similiar).