There are two main types of damage associated with flat irons. High heat leads to heat damage and rusted, chipped or scratched plates lead to mechanical damage.
When shopping around for flat irons, the plates should be the deciding factor. The most effective irons have better plates and straighten the hair easily with less damage. Ceramic plates provide fast, evenly- distributed, constant heat. Smooth plates won't snag hair.
I'm all for savings, but a higher-end flat iron, with better plates, is a smarter deal. There shouldn't be a need for frequent replacements and anything that causes less damage is the wiser investment for your hair.