The wild bird seed (garden rather than pet department) is the most economical bird seed. However, watch out for brands with lots of the large (nearly BB bullet sized) red sorghum seeds. They are more irregularly shaped than the smaller millet which they love. The millet can come in either red or white but is smooth even if often flattened but the sorghum has a point and indentions. The sorghum was actually bred to taste bad to birds but it's cheap so of course bird seed companies often use it. Got to love the pet food industry that does things like package rabbit food as Guinea pig food without the vitamin C they need to survive and at a hundred times the price of bulk rabbit food.