With the initial $5 gig I will only make a simple sound design for 10 seconds of video. You will get the audio file and/or a quicktime. But it depends on the format of your video, if it is really heavy it will be a more expensive job.
I work using a huge sound library I have, I record sounds when needed, I use digital synthesis and some analogue, I use many methods...
With the extras you will get a better quality job, or more duration of sound for your video.
with another extra: the mastering, so the sound will be optimized for youtube and it does not sound quiet with not a lot of volume and the mixing so it sounds more real.
If you send me a reference of something you like the better.