This video goes over the steps on raising a sunken lawn sprinkler head in less than 5 minutes for under $1.

Disclaimer: this video was done on a Rain Bird brand fixed sprinkler head and may not work the exact same for other brands or other types of sprinkler heads.

The item used to raise my sprinkler head was an Orbit brand 1/2 inch cutoff riser that I purchased from the hardware store for $0.51 plus tax.