The etiquette for giving cash is very highly dependent on your ethnicity, and the ethnicity of the bride.
According to mainstream standard etiquette, cash is given under only two circumstances. The first of those circomstances is when a close wealthy relative makes a significant settlement on one or both of the newlyweds. In the latter case, the amount should be enough that, once invested, the income is enough to cover rent and necessities for a modest home. I am guessing this is NOT your circumstance.
The second circumstance under which mainstream etiquette allows a cash gift, is when an employer, house-holder, or distant much-wealthier relative gives a gift to a very-junior employee, servant, or dependent relative. A gift of cash reflects the lack of intimacy in the relationship. If you were intimate with the recipient, you would be expected to spend time and emotional effort selecting an appropriate gift, but that is not something you owe to a distant dependent. It also reflects the difference in income between giver and recipient: since a painless “tip” from the better-off giver equates to valuable capital for the more indigent recipient. The amount given in this circumstance should be on the order of one day’s pay (or one “appointment”) at the recipient’s pay-scale. This is similar to what you probably give as an annual gift at Christmas time to your cleaning lady, newspaper boy, or hairdresser.
In Asian cultures that practice “red envelopes” or equivalent, everyone is expected to give money, but it is still scaled according to the relative difference in income between giver and recipient, with wealthy people expected to give more and less-wealthy people giving less. The absolute amount varies by community, so it’s good to talk to other relatives and friends of the bride and find out what they are giving, and adjust your gift according to your status in that community. If no-one else will share that information with you, then give about half a day’s pay if your income is the same, and scale up or down accordingly for income difference.
In central European folk-cultures where wedding-money is treated as a kind of common-wealth lending pool, the expectation is that your gift will reflect the gifts that have recently been given by the newlyweds’ family members to members of your family. Talk with your relatives to find out what the going rate is in your community.
In eastern European folk-cultures where “presentation” and money-dances of different sorts are practiced, money is given without anyone keeping track: pinned to the bride’s dress or tossed on the dancefloor or poured into a basket, according to the different customs. Since no-one is keeping track, you can give whatever you feel like — again depending on your own income. If you don’t mind eating noodles for the coming month for the sake of the bride and groom having a nicer trip to Bermuda, then give a little more; if the landlord is at your door threatening eviction if you don’t pay the rent, then give a little less.