I think as well that most frames are made by Giant and maeda and a few other makers in Asia. Some makers have a great deal of quality control because they have representatives who enforce quality and acceptance limits for them. O thers may not be so strict. Look at Specialized. I think they have always contracted their frame builds, yet their product is excellent. Others, not so much. I saw a frame at the co-op with such undercutting on the welds that we sent the frame for recycling then and there. Even though I had seen the fellow who owned it commute to work on it for several years. Because there is no insurance quite like good quality. And no liability like poor quality control. Three recommendations, which i have learned from my slight experience
1- Learn what good quality and poor quality look like, so you can tell the difference. Look at the low end and high end of good branded bike production, and look for things in common on both. Or just ask some fellows at the local bike shop (LBS).
2- Try to see what the difference in price among bicycles is caused by. Is it truly quality? or quality of components, hung on the same frame? If so, you may buy a cheaper bike, and upgrade components as skill and finances permit. This will be more expensive, but may be more affordable for you.
3- Realize that mail order, while perhaps giving you a good bicycle, does not offer you the luxury of inspecting the machine before you buy it. Also, some bicycle shops fit you to the bicycle, which is very important to the end result, but which can also be researched online. This research will provide you, as the rider, insights which will be very helpful to your own enjoyment.