I think it depends on how close a friend you are, and who the people married are. However I would give at least $100, and probably more like $150 if I were a couple.
However, as a single, I never know what to give and usually give under $100 as supposedly it is around the cost of the meal.
How on earth would you know the cost of each meal, I know not, as they can vary so very, very much.
I have a couple of Singaporean and Malaysian friends/colleagues. There norm there (from what I can gather) seems to be to give a sizable sum of cash, with the intention being to assist the new couple with starting a their life together, particularly with the biggest challenge - obtaining accommodation. The amount expected seems to align with your socio-economic status (eg. $50 nfrom someone on the poverty line would be seen as a greater gift than $10,000 from a millionaire relative).
It seems to work well over there (at least from my limited observation of my friends/colleagues), but their culture is obviously different to ours. Firstly, their weddings seem to be even bigger deals than they are here, with the collective functions totaling hundreds or even thousands of guests & gifts. Secondly, it is still the norm there not to live together before the wedding.
Conversely, it seems now to be the norm in most western societies to do it in the reverse order: 1) live together (accommodation) 2) start a family (planned or otherwise) 3) think about whether marriage is an option or the right thing to do. In this context, given the couple has already experienced the first few years of their possibly life-long union, they would seem to be more difficult to buy for because they've already done the whole set-up house and baby-shower thing. So perhaps cash or gift vouchers are the more obvious option?
So back to your question of how much? If they're about to live together for the first time (i.e. it's their first marriage and cohabitation), it should be more than if they've already being cohabitating or it's not their first marriage. My view is that, in the latter case, it's more just big party that an established couple is throwing albeit under the guise of a "we're starting our our life-long union together" (wedding) situation - even if the reality is that that union started long before, so it really is just a really big version of "would you like to come over for a BBQ this weekend" - in which case you usually take a bottle of wine or something (so in the form of a wedding, the gift in this case would be of similar proportion/significance - i.e. much less than the cost of your seat at the event).
On the other hand, if the invitation is to a wedding in which the couple is literally starting their long term commitment, cohabitation, and potentially a family, I would argue the amount to give should be considerable but according to your SES means. For the average person, the figures already proposed in the majority of responses in this thread would seem reasonable (EG. $50-$150 for a friend or colleague, more if you are a relative, and more again if you are immediate family).
$100? We're going to a nephews wedding in Bali in 1 & 1/2 months, & as the couple live there, I wouldn't think they'd want to be lugging wedding presents home again, should they return to Australia, so that & having us fly over there, I think is a good enough gift! When another nephew married in India, 3 years ago, & were then returning to Australia, they'd asked for BIG bath towels so we bought them 2 sets.