A number of the larger paint companies have a line of "student grade" oil paint. Student grade paints are made with cheaper substitute pigments and inexpensive extenders. This allows them to be sold for significantly less than "artist-quality" paint.
If you are just starting to learn oil painting, don't buy the student grade paint. It's hard enough to learn how to work with good oil paint, let alone handicapping yourself with the cheap stuff. Student grade paint doesn't handle very well and the substitute pigments are of much lower quality. Because they use extenders and fillers, the paint doesn't go as far.
If you don't have a lot of money, buy fewer different colors of good quality paint. Get earth colors, ultramarine, and other pigments that are inexpensive. You can do a lot with just five or six tubes of paint. Add more colors as you can afford them.
Robert Doak makes very high quality paint that is quite affordable, by the way.