cheapest way to get rid of clover grass on lawn?

Clover loves a hungry lawn - it makes its own nitrogen, so it wins when the grass is short of it. Feeding the lawn properly in spring and autumn does more long term than anything. Mow a bit higher too, longer grass shades it out. Rake the patches upwards before you mow so the runners get cut. For small patches dig them out by hand. A selective lawn weedkiller that lists clover on the label works for big ones, but spot treat. Or leave a bit, the bees like it.