Alcohol content is not the primary driver of the cost of beer and so unfortunately there is not formula for directly mapping the price of a beer to its alcohol content. There are a number of ingredients in beer, and the non-alcohol producing ingredients (hops and other flavoring ingredients) can easily cost as much or more than the ingredients that produce the alcohol (malts, adjuncts, other fermentable sugars.)
This is why some extremely high alcohol beverages (malt beverages, cheap vodka) can cost far less per ounce than a high quality, considerably lower alcohol beer.
HOWEVER: If you're not looking for a general formula, but want to compare two specific beers, the formula would be to multiply the ABV by the unit of measure for the beer (ounces here in the U.S.) and then divide the results by the price.
EXAMPLE: So I have two beers, one that is 12oz and 4.9% ABV, and costs $0.83 per can, and a second beer that is 16oz and 9.2% ABV, and costs $3.00 per can.
For beer one I multiple the alcohol times the quantity 0.049 * 12 = 0.588 (It contains 0.588oz of alcohol per can.) Now I divide the price ($0.83) by the alcohol content (0.588) and I find that the first beer costs $1.42 per ounce of alcohol.
For beer two I multiple the alcohol times the quantity 0.092 * 16 = 1.472 (It contains 1.472oz of alcohol per can.) Now I divide the price ($3.00) by the alcohol content (1.472) and I find that the second beer costs $2.04 per ounce of alcohol.
So, in this specific case, the lower alcohol beer delivers more alcohol for the price. ($1.42/oz vs. $2.04/oz.)
GENERAL FORMULA: Units are as you give them (Use price in dollars and volume in ounces to get $/ounce -- Use price in Euros and volume in milliliters to get €/ml -- etc.)
Price of alcohol content = [Price of package of beer] / ([Units per package] * [Volume per unit] * [ABV])