R.D.
I would think $100. If they chose to have a destination wedding where everyone has to travel to get to them then I would think lesser is fine. Also depending on the state they are getting married in-NY and MA are more expensive than other places. I think the general rule of thumb is that you want to cover the cost of your plate. But like you said you've already spent $700- Whatever you can afford is what you should do. Your friend wants you there to share their special day-they shouldn't be looking at the gift.
Have a great time :)
R.