Depending on how handy you are, it still could be a diy job. See the attached picture. Our yard was too steep to play in as well. 110' long retaining wall. Originally the yard sloped all the way down to the curb in the picture.
$1,000 in bricks, $300 or so in gravel, less than $100 in plumbing and appropriate drainage. Dirt was free (thankfully my dad and brother had some available, plus trucks to move it). Tractor was borrowed and thus free.
Add in lots of manual labor by me and a few hours of help from friends/neighbors. Start to finish of the wall was less than a week.
Permits here are required if the wall is over 4'. Mine is just under of course. Lookup "french drain" for pictures on how to appropriately drain. Contact city, etc on local permit regs if any.