A knockoff purse is exactly what it sounds. It is a replica of a major designer bag, but has one tiny thing different. If the knockoff is Prada, then it might be called Proda. With that tiny change, they are allowed to sell the purses legally.
Usually knockoffs are much cheaper than you will get a get a real designer. In New York City, it is the central for fashion. Many designers are located in either New York or San Francisco. If you purchase a knockoff in New York City, you will most likely buy the purse from a street vendor or in the markets.
If you purchase one from a street vendor, you are most likely going to pay less than the markets. Why? The main reason why they are cheaper on the streets than in the markets is because street vendors are prohibited to block the walkways with their merchandise. They have to set up quickly, make as many sells as they can before a member of the PD comes around. Then they move and set up somewhere else.
This all happens usually within twenty minutes. In the markets, they have a stable range of customers so they don't have an urgency to sell. However, you should watch the street vendors. The best part of purchasing a knock off purse is the haggling. A designer purse could go for $250, but you could get an exclusive knockoff for $20. When haggling for the knockoff let the vendor start off and then you have the power to lower it. If you see they aren't going for it, start walking away, they will usually cave in once they see you turn away.
Are knockoffs just as good? The answer is yes, for the price for you pay. If you pay $30 for a bag, expect it to be worth maybe $20. Meaning, they aren't as great as designer and may fall apart, but you never know when you might get a steal by buying a knockoff. The whole point to the knockoff is to allow normal, everyday people to owe "expensive things", well the trick is to get your friends to think that you have expensive things. Knockoffs make you look better, as long as you don't let anyone realize that your bag is a knockoff.