Cheapest to own is probably the WR250R or X. It is slightly quicker than a ninja, has no body work to screw up in a drop (just plastic), weighs less, has less than half the engine maintenance, and handles better... but they also cost more
to buy and you wont find any for under 2 grand.
DRZ400, similar to the WRX but faster and requires more maintenance. You may be able to get a super cheap one though and it will definitely be more powerful than either 250, also a lot less gutless... BUT they are known to be buzzy on the highway. It bothers some.
Neither of those are sport bikes though, they are supermotos and dual sports. Meaning less wind protection and different styling that some love and some hate.
I personally commute on my WR250X. With the constant stop and go traffic it still manages 60mpg, it costs me roughly $5 a day to commute in my light truck vs $1.60 to do it on my bike. Also, the difference in insurance price would allow me to buy a set of tires every few months ($30 for the bike vs $120 for truck), except I still own and insure the truck
Have to buy groceries, haul the garbage off, drive to work in freezing rain, etc... so I need it.