An 8lb keg will load around 2,300 rounds depending on the powder charge. I used 24g's to come to that number.
Powder will run around $120-$150 for 8lbs unless you can get some military surplus powder which is around $90 for 8lbs. Primers will be around $25 per 1k again depending on the brand.
You can buy cheaper primers like Tula or Wolf at a much discounted price. $15-$21 per 1k