GoogleDietician says that a pound of 90/10 beef produces about 12 ounces of cooked meat, and a pound of 80/20 beef produces about 10 ounces of cooked / blotted / rinsed meat. If you pay 17% more for 90/10, the 80/20 is cheaper. Even if those numbers are really 13 and 9 (more in favor of lean), a 30% price differential means you come out ahead with cheap meat.
The USDA (
http://www.ams.usda.gov/mnreports/lm_xb459.txt) says the current (sales within 21 days of 8/3/07) wholesale price for a 10lb chub is:
Ground Beef 73% 99.94
Ground Beef 75% 108.39
Ground Beef 81% 113.55
Ground Beef 85% 140.76
Ground Beef 90% 157.59
Ground Beef 93% 173.49
That makes 90/10 about 38% more expensive than 81/19 is, at the wholesale level. So you'd need a store that was willing to sell 90/10 at a significantly lower profit margin than they get on 80/20 to come out ahead buying 90/10.