Cheap skillet against an expensive one: what actually differs and what is the same

The iron is essentially the same. What you pay for is finishing, consistency and weight distribution, and only some of that reaches the cooking.

Genuinely different

• The cooking surface. Expensive pans are machined or ground smooth. Cheap modern ones come out of the mould with a pebbled texture. Smooth releases food better from day one and seasons more evenly. This is the one difference you feel every time you cook.
• Wall thickness consistency. Cheaper casting varies, which produces hot spots. On a gas ring you will notice; on induction, less.
• Weight and balance. Thin and light heats fast and loses heat the moment food hits it. Heavy holds temperature, which is the entire point of the material for searing.
• Handle and pour spouts. Comfort and mess, not cooking.

Not different

• The metal. Both are cast iron and both will outlive you.
• Seasoning behaviour after some use, since a pebbled pan smooths out with time and cooking.
• Heat retention at equal thickness, which is physics rather than brand.

The cheap route that gets you the expensive result
An old pan. Skillets made decades ago were routinely machined smooth as standard, and they turn up in second-hand shops for very little. Rust is cosmetic and comes off; a crack or a warp does not. Check it sits flat on a worktop and check the rim for hairline cracks, and everything else is fixable in an afternoon.

If buying new: pay for smooth, not for the name. Below that, the cheapest pan and some patience gets to the same place slower.