I think it depends a little bit on where you are. If you are in the capitol district of MD, I'd consider that a high cost area, and $50 would be the lowest I'd give.
If you're in the more rural section, somewhere between $30 and $50 is reasonable.
I know some people have different opinions on this, but if I'm unsure, I base it on the how much the family spends for me to attend. For instance, if it's a church wedding with a cake and punch reception, $30 is probably enough. But if the reception is a sit down dinner with open bar and DJ/band, I'd think that $75 would be the minimum I'd give.
Just my opinion, and I know some people feel differently.
Have a good time at the wedding.