Cap't Kirk,
You are correct, M-1 costs about $5/qt at regular price. Wal-Mart has it in 5qt jugs on sale for ~$18 now & then, but it's REAL hard to find the 15W-50 (red cap) variety. However one big advantage to synthetic is the ability to use extended drain intervals. You can use dino and change every 3-5kmi, or use synthetic and go 7.5-10k easily (longer with analysis). That alone can drop the cost in half, or even make it cheaper than dino depending on when you normally change it. I use Delvac-1 which is a little pricier (~$21/gallon), so my changes are typically $50 every 10kmi ($42 for oil and $8 for an OEM filter.) I'm going 10-15kmi and having the oil analyzed with each change via AccuTrack. That drops me to one change per 12-24 months, depending on the car...!
BTW, where is the leak you're trying to chase down? I have to say M-1 didn't CAUSE it, however it may have made it visible when it wasn't before!
Regards,