See, I quaver a bit at spending a lot on weddings.
Yes, the bride and groom often pay $60+ a head on food and drink, but that's their choice.
I've been to lots of different weddings (formal sit-down dinners, beach BBQ, spit-roast and buffet salads) and all have been awesome! So if the bride and groom choose to spend a lot (or a little) on their own wedding that shouldn't dictate how much you should spend on them for a gift.
I say $50 if you don't know them well (or don't socialise with them on a semi-regular basis) and around $100 if you are great friends/family.
Especially if you're having to travel and cover all the costs involved.
You're choosing to go (it's not the law to attend every thing you get invited to) so I think you should get to choose how much you spend on gifts.
That's just my two cents