website programming

The website must accomplish the following requirements:
Description:
This website is for people who are sharing their expenses. Example:
A group of 4 flatmates, that have to share their common expenses. We have A,B,C and D members. For example, A buy bread; then he register his expense. B buy drinks and fish; then he can register it. C buys milk, but D is allergic to lactose; then it register this expense but ticking D name for excluding him.
When they want, they can click a button (or link, whatever) where the expenses were compensated. The algorythm generates a PDF file showing who debts money to others and how much. (Consequently who receives money). It shows in a table and in human language, I mean: "A has to pay x to B and C"
This PDF contains the expenses of every one separeted in different pages and tables.
How it works:
x People can register themselves. Login password...(you know it yet). The can create groups and invite other people to it: for example "Flatmates" "Party 2011" (It has general purpose, then it can work with small ideas).
x In these groups, each one can register their expenses (erase, modify...only their own ones). And this expenses are mainly for everyone in the group, except if someone is excluded; then his name has to be ticked before registering.
x When someone want, he can balance expenses.
Requirements description:
x Users management
x Message management
x System of adverts, where people can upload pictures and information (This website has also a system of searching shared flats).
x Group management
x Expenses management
General overview:
x This system is for every user, it has to be clear, very very clear and simple. Not complicated CSS.
x It is for everything related with shared expenses, flatmates, uploading advertises where people shows that they are looking for a flatmate.
x Fast GUI, do not ask for many details for registering users. The minimun, no street, and all these things (not the same for flat adverts. It should have a good information, not excessive. It should be connected to Google maps also).
x Avoid SQL injections, server flooding and all these stuff. (I suppose it is obvious).

Please, tell me what would be the price if you code it again (I am afraid of recovering files, I think this is our best choice). I want efficiency. You can use free CSS, CMS, and of course you can reuse all typical functions: User management,, uploading pictures...

You can ask me whatever, and i can send you some functions if you want to. I send you a diagram of the database if you want or need it. It is easy to design, but you can ask me for things for delivering you from working in things already done. But mainly, think that you are starting from zero (I mean, you can use every free function that already exists, but we havent here the project ).

If you want me to make diagrams, ask me.