The Iron Cross 2nd Class is the first level, the "entry level" of the

Form
Visually, the Knight's Cross and the Iron Cross 2nd Class are nevertheless very similar. In both cases, it is a black cross with a silver border.

Strap eyelet
There's a ribbon loop on the 12 o'clock arm. On the Iron Cross 2nd Class, this loop is circular and closed.
On the Knight's Cross, it is oval and open. The ring (called the spring ring) and the cross can be separated.

Size
Another difference is the size of the crosses.
The Knight's Cross is always at least 47 mm wide and high. The Knight's Cross ribbon is usually 45 mm wide.
The Iron Cross 2nd Class is generally no wider than 43 mm. The ribbon of the Iron Cross 2nd Class is never more than 30 mm.

The Iron Cross 1st Class

The Iron Cross 1st Class is easily distinguishable from the other two. It was worn as a pin-on decoration and therefore has a pin or screw plate on the reverse for attachment to the uniform.

Value

The Knight's Cross was awarded approximately 7320 times, making it much rarer than the Iron Cross 2nd Class, which was awarded approximately 4.500.000 times. Therefore, its value and price are also considerably higher. The price of a Knight's Cross is approximately €6.000.
An Iron Cross 2nd Class is typically worth around €50. Rare variants or those in exceptional condition can fetch significantly more in both cases.

Order of the Iron Cross.

The Knight's Cross is a higher grade of the Order of the Iron Cross. Its award required possession of the preceding grades: the Iron Cross 2nd Class and the Iron Cross 1st Class. Therefore, every recipient of the Knight's Cross was also a recipient of the Iron Crosses 2nd and 1st Class.