According to the Sonic website, you need a site of around 2/3 of an acre to open up a "traditional" Sonic, which is just shy of 30,000 square feet, or a site that is about 150 x 200 feet.
Now, I'm not sure what the rent on 30,000 square feet in your run-of-the-mill shopping center in Fairfax County is, but I'll bet it's a pretty penny. I saw they're offering 1 acre between Bailey's Crossroads and fucking Culmore for $175K per year.
If you want to buy the land outright, there was a site offered up at the Burke Village Center that would have been about the right size. The price was $1M. I would imagine this is probably on the low end of what you'd expect to pay, because the location is kind of shitty.
You need to have at least a $1M net worth and liquidity of $1M to open 2 Sonics, which is the smallest number of stores they will let you open, and I think it'd probably cost you close the high end of their estimate of $710K - $3M.