Eh, guys before me have brought this up, but you absolutely get what you pay for. If the occasion calls for a designer in the first place, might it be worth the $300 to get some decent talent on the job?
If it's not worth it or not in your budget, you could use free vector graphics and/or do it yourself. All you need is to download some open source software and read a couple tutorial blogs.
You really shouldn't. Depending on what your priorities are, do one of the following instead:
If you find volunteers: Don't pay them even if you'd like to, because you want to find someone who really just wants to do it. This also means they'll do it by their own rules and you'll have to accept. (In my case that involved hanging around for hours in the middle of the night watching some friend of a friend of a friend testdriving his brand-new Adobe software and then waiting weeks for him to send me the actual file)
If you're hiring a professional, "good" is - in my experience - quite independent of the price tag, although good professionals are rarely cheap (they are surprisingly often medium-priced) and always worth what they charge you. I went into more detail in this answer.
Call up your local college, or art and design colleges that may not be local. Get the name of one of the 100 level professors and have them forward your request onto the students. Students are cheap, and often the professors know which students have a good talent and work ethic.
Alternately, decide where you're going to flyer, and check out the flyers already going in that distribution channel (bus stops, light poles, etc). Find a few flyers that are decent, contact whoever posted them and ask who they used.
Lastly, Craig's list it or use some other local willing-to-buy listing service.
I've used oDesk to outsource graphics design work on a budget. You can see someone's portfolio before hiring. Sometimes it has worked out really well for me, sometimes I get junk. For the price (many designers in poorer countries are charging $5-$10/hr), it's worth trying.
For really cheap stuff you can try the Digital Point forums:
1- Simple is a subjective term. If its really simple then you might not really require expertise of a real graphic designer.
2- You should estimate how long do you expect a real designer to spend time on your flyer and then how much are you willing to pay an hour. In another words I don't think you can get much quality creative with $50-$100.
3- Lastly I recommend you try your local Kinkos branch.