If you measure your lawn in square feet/metres, the bags of gravel at your local friendly DIY store should have written on them the area one bag will cover. You can then work out how many bags you'll need.
It rather depends how thick you want the gravel. Assuming 30mm thick, one of those Bulk Bags that Builders Merchant's supply will cover about 15 square metres of area. Unless you have a very small area to cover, bulk delivery of large bags is liable to be the cheapest way to do it. It depends whether you are after fancy colours.