Different companies are going to choose different compromises to make to hit their market segments. And different riders are going to prioritize certain factors more than others. Certainly weight factors into a lot of the upgrades, as well as things like ride quality, braking, and drivetrain performance. But here's what I've generally noticed in the road bike segment:
- Cheaper bikes usually have heavier aluminum or "no name" chromoly steel frames
- More expensive bikes will have lighter aluminum, steel, or carbon fiber frames
- The more expensive frames tend to offer a balance of stiffness for performance and compliance for comfort in key areas
- Often aluminum or steel forks on cheper bikes; carbon forks up the line
- Cheaper bikes often have off-group cranksets, usually entry level FSA
- Brakes are usually basic Tektro calipers or rebranded FSA instead of nicer groupset brakes
- Hydraulic disc brakes are usually a premium component
- Cheaper bikes have basic, heavier wheelsets with cheap OEM 30tpi tires
It seems like one can get a lot of bike for around $1000, but there are still meaningful upgrades from there to the $2500-$3000. And if you want an electronic groupset, expect to pay even more. I've never experienced electronic shifting, but it sure seems like a nice luxury. You can keep spending more and continue to experience performance refinements and weight savings, and I can't tell you where to draw the line. But I think most riders can be perfectly content riding an "entry level" $1000-$1500 bike.