The answer is simple. The one who needs the other more pays. If both need one another equally, they collaborate for mutual benefit.
Let's look at several examples.
Let's get the obvious one out of the way. If there is a client, the client pays both the model and the photographer.
In the real world, it's typical for an aspiring model who signs with an agency to pay for a test shoot. He/she will typically pay anywhere from $400-over $1000 depending on the market. This cost is incurred by the model, not the agency. In some very rare cases, the agency may front the money and later deduct it from the model's earnings but that is a rare exception, not the rule. In other cases, a model may be able to find an agency qualified and approved photographer to test for free if he/she has a look that the photographer needs. Again, that is an exception and not the rule. In the agency testing world, qualified test photographer rarely pay for models. Why? Because they don't have to. They have an abundance of models to choose from. Sometimes, they may choose to pay a particular model for a particular look that cannot be found otherwise.
Another example where a photographer may pay a model is for a personal project such as a gallery show or a book. In these cases, the photographer becomes the client. The reverse is also true. A model who needs pictures to update her subscription site might pay a qualified photographer to shoot sets. Here, the model becomes the client.
Now let's address the whacky world of Internet modeling. The popularity of Internet modeling sites like Model Mayhem and One Model Place where there is no barrier to entry has resulted in a mentality that no one pays anyone for anything. There is nothing wrong with collaboration for mutual benefit. Artists have been doing that for centuries. The problem arises when one or more party feels a sense of entitlement. A "club hot" girl who gets told by every half drunk guy at a bar she should model might start to believe it and open an account and start demanding pay. There is certainly nothing wrong with that if she can get it. On the flip side, there are plenty of "photographers" who claim 30+ years of experience shooting crap who demand pay. Anyone can demand anything, the question is do they get it?
Generally, in the Internet modeling arena, hobbyist photographers pay hot girls to model for them. One can argue if those are real photographers or real models. That's not really the point. If they can get paid, more power to them. If you hang around modeling sites long enough, you will see a certain pattern among the popular models whose clients are mainly photographers. They typically start out locally doing shootouts and/or one-on-one shoots with photographers. At some point, they will exhaust the local supply of photographers willing to pay them and they become traveling models. In essence, they are following the work.
It's very rare for a photographer on a modeling site to make money shooting models. It's more common for models on these sites to make money charging photographers. Why? Because many members calling themselves photographers are horny men and many members calling themselves models are young hot girls. It really just comes down to that.
Of course there are exceptions. There are quite a few art models who shoot with a few artistic photographers but I find most of those relationships are usually not driven by money.
Bottom line, if you are a half decent looking female model willing to get naked, you'll be able to make some money charging photographers for their personal projects. If you are an agency qualified model looking for commercial client work, look elsewhere. You're not going to find much on Model Mayhem or One Model Place. If you are a talented photographer looking for model for collaboration, it'll be smooth sailing for you. Art photographers and art models also tend to do well finding one another on modeling sites.
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