How much does 500 sunflower seeds cost?

That depends on the variety you get and where you buy them. Oil-seed from a feed store would easily be the cheapest, but they are sold in bulk bags by weight. A 5 lb. bag would probably cost about $8 to $10. Buying them as seed packets from a garden store would be much more expensive, probably costing as much as $100 for 500 of them, assuming ten seeds per packet (many seed packets are also sold by weight).