Pattern: shirt onesie free pattern and tutorial. I altered the bottom pieces to have lots of gathering.
Fabric: women's button up shirt, hand-me-down
Time: 2 hours, I had to alter the pattern and then figure out the construction as I went.
Cost: $.25 for snaps and thread
If you want a simple romper for a 9 mos old baby, check out this free romper pattern and tutorial.