Ego-C Twist would be a pretty good device, $30-$35 for a battery and charger... Variable Voltage, 510/ego threading so tons of choices for cartos/attys/tanks. Although it's a new device, Joyetech's reliability track record has been pretty good, and would be my first recommendation for any person who wants to jump to variable voltage.
I also picked up a Big Kazoo from Kalamazoo Vapor Shop, it's a pretty nice 5v regulated box mod for about $35, add some AW batteries and a good charger you'll probably be around 60-70 for a well built box mod... 5V is pretty much where I like my SR Boge cartos, so works really well, don't have to worry about voltage drop as it's a regulated mod.
Or there's always a lavatube... Good kit will run you around 70-80 for new higher amp unit, 50-60 for the original lower amp unit (works great for 2 ohm or higher devices, get the new one if you want to go lower then 2 ohms)... Or if you're willing to spend a little more, a buzz pro or infinity pro are great devices for the money....