Have to agree with Gapton and Steve here...your client is trying to swindle you, no question there. They used you for free, and are not trying to get YOUR product, the actual photography, for free. A client has no reason to need or want a RAW file...they should be paying for a professionally finished product.
Tell them NO! Its the responsible, professional thing to do. You can't be a professional if you work for free and give your product away for free. If they ditch you for being too stingy, well good riddance! Find another customer who is reasonable, honest, responsible and timely with payment, and who does not expect everything to be free. Have a written contract between you and every client as well, to give you some legal power to ensure payment when the time arrives.
I also recommend keeping track of how your clients are. As a professional you should expect nothing less than to be paid for all of your work, in full, and on time. Not every customer is going to be a trustworthy person, and there are a lot of people out in the world who will stoop to any level to serve themselves as cheaply as they can. If you encounter clients who are regularly trying to get your work on the cheap or for free, or who ask for RAW images (thats like asking a film photographer for his negatives), its a side duty to remember who they are. Warn any other professional photographer friends you may have about them if they happen to work for (or have an opportunity to work for) the same people. Keeping such records gives you something to fight back with if a customer tries to use the fact that you refused to serve up your RAW versions without payment.
Don't sell yourself short, even if you have not done exactly the kind of photography a customer is asking for. Take a few sample shots of their event and show it to them, but don't give them everything for free. You might also try taking a few shots of a similar event ahead of time to boost your portfolio, and demonstrate your ability up front so you don't have to give the customer anything for free. As a professional, you have a right to sell your skills and your product for a fee.