You don't have to be a top, top girl to make money as a model but you do need to commit to it full time. There's so much travelling involved, you can't usually hold down a second job. For shooting one catalogue you can make $30,000+ (USD) minus the agency fees (up to 30%). Not shabby for a week of work! Certainly more than a lot of people the same age make in a year. And of course travel expenses are normally paid for by the client (your agency should negotiate this).
Expect to do a lot of editorials in the beginning for free or for peanuts ($100 or less). You need the tear sheets to get better jobs (advertising work) in the future.
Hope that answers some questions here without getting too redundant.