What's the most affordable way to level a sloped yard?

The most affordable way is usually to regrade the yard using a cut-and-fill method rather than trying to make the entire yard perfectly flat. Remove soil from the higher areas, move it into the lower areas, compact it in layers, then finish with screened topsoil and seed or sod. Keep a slight slope of about 2% away from the house so water drains properly. If the yard has a significant slope, it may be cheaper and safer to create two or more level terraces with a small retaining wall instead of moving a large amount of soil. For a small-to-medium yard, renting a mini skid steer or compact loader can also save a lot of labour compared with doing everything by hand.