My bike was about $600 new. I keep it in the garage at home and locked up to a rack at work. Bike theft is very low in this area, so a simple u-lock around the frame and wheel does the job.
Would I hesitate to buy a more expensive bike? No, but I would never leave it unlocked anywhere. That's just asking for someone to steal it.
Why spend more on a bike? Higher quality parts and a better ride. Why buy a BMW when a Kia still gets you from A to B?