Imagine a small goal, say you sell ten pairs of jeans for $120 each, that's $1,200. Work back from that to where you are now.
You'll need to research design, sales avenue (ebay sounds good, why not), materials, manufacture, total costs... there's some math in there, but not as much as starting your own line.
However the scale is small enough that you can hold it in your head and (worst case scenario) if you don't sell any then its not the end of the world.
Designing, making and selling 10 pairs of pants would feel pretty good I'd say. If that's not enough motivation then you should find another hobby, mate. After the first 10, you'd know what parts of your process you need to improve on to sell, say 20 pairs and make some money at the same time. Eventually you can start farming out parts of the process, ie, pay someone to make the jeans, or find a shop to display them for you etc. At the start it's important you get your head around the whole process and like homeless-joe mentions, know the numbers a little.
True story: when my wife started her gospel academy the first thing she did was a trial project to see if it would fly. She rented a studio and marketed 8 lessons leading up to a performance concert with the proceeds going to charity. She made the money for the charity, but what she was really studying was how to market signing lessons for groups of fifty, how the numbers would stack up and how to retain students long-term.
After that, when she opened her school she was doing trial lessons where 40 people would come for free, and after the lesson decide whether to join or not. Basically a mass sales-pitch. She had about 15 trial lessons over 6 weeks with a target of 150 students. First lesson, 2 of 40 sign up. Second lesson, 3 of 45 sign up. Then 3 of 35. Not good, but her team changed a few things around and eventually fixed the problem. The remaining 12 lessons averaged 18 sign-ups and she was on her way. The point is, as homeless joe mentioned, that she hadn't committed any resources until the system was working and she understood the math. That's why we're suggesting making your own stuff first. If you succeed there then you'll be able to have a clothing line eventually and you'll understand all the pieces to the puzzle.
« Last Edit: April 01, 2009, 08:43:19 PM by underclass »

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Loaded-Gun.com - I don't know what the hell they are talking about or why they are even there. They don't make serious points and they don't joke, but they still manage to make a lot of posts somehow.