Here is one measure you can use. The product of price and GPHM gives you dollar gallons per hundred miles. The higher the price or the more gallons per hundred miles the larger this number will be. Larger numbers are worse. So to get a measure where bigger is better, we can take the reciprocal. For example, a Toyota Prius sells for about $24,000 in its basic no-frills version. Overall (combining highway and city) it uses about 2.27 gallons per hundred miles. So we rate its price-mileage performance by dividing 1 by the product of these two to get 0.0000184 in units of 100 miles per gallon per dollar. For another example, a Honda Fit costs about $15000 and uses about 3.13 gallons per hundred miles. It comes out with a rating of 0.0000213 in the same units of 100 miles per gallon per dollar. The Honda Fit costs less but has worse fuel economy, but if you are interested in fuel economy per price, the Honda Fit comes out ahead.
This measurement gives numbers that are hard to understand. Instead of giving the raw number, we could compare each vehicle to a standard vehicle. Taking the Prius as the standard, we would calculate each car's rating and divide by the 0.0000184 value of the Prius. Then results larger than 1 are that many times better than the Prius and vice versa if it is less than 1. So for example in comparison to the Prius, the Honda Fit comes in at 1.16 or 16% better than the Prius at delivering fuel economy per dollar of price.
Here is another way of comparing the fuel economy you are getting relative to the price. Say a typical car will have an operating life of 8 years and it will get driven 15000 miles each year. That car will thus cover 120,000 miles over its service life. Then using its fuel economy we can see how many gallons of gas it will use. Multiply that by your estimate for the average price of gas over the next 8 years to get the total lifetime fuel cost. Now compare that to the price. For example, for the Prius at 2.27 gallons per hundred miles, we see it will use up 2724 gallons over the 8 years. Let us assume the average price of gasoline over the next 8 years will be $4.00. I actually think it will be a lot more than that but let's go with $4.00. That means a cost of $10,896 in gasoline over the service life. Dividing that by the price gives about 0.45 or 45%. So in this operating scenario the Prius has gas costs at about 45% of the sticker price. Repeating for the Honda Fit, we come up with a fuel cost ratio of 1.00 or 100% of the sticker price.
With this second method, we can generate an overall rating for a vehicle by adding the sticker price to the scenario total gas price. The lowest overall price is then the best deal. Here we find that the total price for the Prius comes out to $34,800 and for the Fit to $30,000. So again we see that according to our rating score, the Fit gives the best gas mileage for your money, although it has poorer absolute mileage. However, the price is sufficiently lower to compensate for that and give it the win.