I'm a big fan of Sonic drinks. Love 'em! If you get one between 2-4pm, they're even fairly inexpensive, as they're half price at that time each day. But I've found a cheaper way to get the same yumminess at home.
I'd mentioned last year that I found coffee syrups at Sam's Club. Each bottle is less than $5. The bottles I bought last year just ran out, so I bought more this week. (Seriously, a very good value, IMO.) I have vanilla, raspberry, and coconut.
Mix a soft drink with whatever combo of syrups you want (I mixed raspberry and vanilla into my Sierra Mist), and voila! A drink like you'd get a Sonic (of course, without the special ice) for much less and without the drive.