The absolute cheapest way would be to just grab a trowel and dig out around the stumps and roots enough so that you can cut the stump short enough with a chainsaw that you can just fill over it with dirt. The stump will rot faster this way and you'll have immediate level ground, but obviously the ground there will continue to sink over time as the trunk rots and you'll just have to add more dirt every couple of years to fill in the sinkhole if you care about the ground being level.