I second Nathan's recommendation. It's best to talk with them and open a discussion for what they are expecting to pay and what you would be willing to accept. From personal experience sometimes giving a fixed price without letting them know they negotiate can often lead to your developer finding someone cheaper if it's over what they expected. Specially if you haven't given them the indication that you are flexible and open to negotiation.
You can say, I typically chage - $, but I am willing to negotiate if you don't have the budget to meet my usual rates.
I have used both a per sfx and per hour rate but it really depends on the client and your experience. I recently worked on a project where we were prototyping and so rather than charge a per sfx rate, we agreed on an hourly rate. However before entering this kind of agreement, clients usually would like some sort of proof of experience so they know you're not going to waste a lot of time and not hit the mark in both stylistic and quality consistency.
I have also offered a trial fee for a first (usually smaller) game so that the developer can test the waters with your skills with the understanding that you typically charge more once the relationship is developed and they take this into consideration while working out their next project's budget. This good will can do wonders for generating strong long lasting relationships.
This is definitely one of the areas you will get better at over time with more and more eperience to read clients and create safe open discussions about expectations.
