One More Mile has really funny running shirts...there are like 10 that I want, here is their website.
Shirts are 24-26 bucks each, which is more than you said you wanted to spend, but keep in mind they are technical shirts, which your friends will appreciate (light, breathable, moisture wicking, fast drying) for future training. Otherwise, they have hats that are 15 bucks each that have the same sayings. Also, stickers, magnets, etc. which are cheaper.
Some examples of sayings are:
running is my happy hour
running is cheaper than therapy
26.2 miles and still smiling
will run for beer (also have chocolate, wine,ice cream, margaritas, etc.)
26.2 miles: been there, run that
Does this shirt make my butt look fast?
Slow is the new fast
In my dreams, I am a Kenyan
Toenails are for sissies
I know I run like a girl, try to keep up
My sport is your sports punishment
(front) one more mile logo (back) Dear God let someone be behind me to read this
I'm only doing this so I can post a picture on facebook
I thought they said "rum"
I'm not slow, I'm getting my moneys worth out of the entry fee
You think I run funny? Wait till you see me walk tomorrow
And my personal favorite:
If you see me collapse, pause my Garmin
Hope your friends have a great race! Yay running!!!!