Hmm, if it's photos you're going to be making money out of, selling her image on, I'd expect that to be a higher rate than if you're just wanting practise. Much like how photographers usually charge more for commercial use.
Her body is her trade (that sounds wrong but I can't word it properly), she's got to make enough money to cover a healthy diet, likely a gym membership/other fitness classes if in good shape, possibly hair products, makeup, nail products, possibly manicures and pedicures, waxing. I doubt anyone wants a model with stringy hair, broken nails and hard skin on feet with hairy legs - right or wrong as you may consider the social conventions. Usually certain foundations are needed to look good with flash. For a fashion shoot, if it's one where she supplies the clothes (rather than set ones), those have to be bought.
I know people do shoot models who don't fit into standard beauty ideals, but I doubt it's as common.
I don't know about 6 months experience. I'm assuming like most professions, some people doing it for 6 months could be better than others with 10 years.
I'd also assume a higher price helps defer perverts just wanting to be alone with a half naked attractive woman.
I'd also expect to pay more if she's working in less pleasant conditions (mostly outdoors) than a studio etc.
Sorry to be "that person", but people constantly moan about photography being devalued. Maybe expecting the same price for a model as 20 years ago is devaluing them, too. Especially of course, if she's good.