Some of the popular items include airbrush paints such as Vallejo Air, model paints, any craft store acrylic paint watered down, and interior house paint also watered down.
I lean towards house paint samples for my primary color because they run $5-7 US and are 8oz which gives me enough for a full suit plus plenty extra for future updates or touchups.
Secondary depends on the color needed and where it's available, but craft paints and airbrush paints are mainstays in my craft space for secondary and tertiary colors.