But thats 75W
My laptop charger draws more than that. Don't get me wrong, handy for many things, including most small flat screen TV's, as well as DVD players. Once you get to 'useful' levels, they get expensive, and understandably so. Not only the larger components for higher power, but also the efficiency must be increased so i doesn't kick out 100W of heat (which can be usefull I suppose). I can see the appeal of a self build, as it *can* be significantly cheaper for 500W+ given access to various scrap parts - but even engineers with years of experience will spend a LONG time designing, measuring, and tweaking. So like everything in life, its a trade off between, time, effort, and money.
I must say though, I never like to 'put off' people starting a project, I'm always up for some good 'ol DIY - alas, with power >200W, its just not worth it in my opinion. When you ad dup the cost of the components, and any measuring equipment you need to beg/borrow/steal, as well as the time taken, and effort in building it, a pre-built one, of any power level, pays for itself. Unless the OP has access to several 'faulty' power converters/battery back up supplies, I don't see it happening, even if he has 30 years experience in the field.