What "requires modification" actually costs you
The label is one line in a listing. The work behind it is not, and the price of the part is rarely the whole price.
"Requires modification" is a single line in a fitment listing, and it hides the widest range of outcomes of anything on this site. It can mean ten minutes with a file. It can also mean a bracket that does not exist yet.
The listed price is the part. What it does not include is everything else the job needs.
Hardware is the usual first surprise. Spacers, longer bolts, a different mounting plate, an adapter — individually cheap, collectively capable of adding a meaningful fraction of the part's price, and frequently not discovered until the part is in your hands and the old one is off.
Then there is the work. Drilling or cutting bodywork is irreversible, and getting it wrong means buying the panel again. Wiring modifications on a modern bike can wake up an error code even when the change is electrically sound. If you are paying a shop, labour on a fiddly fitment can exceed the part.
Two costs are easy to forget entirely. Resale: an irreversible modification narrows your buyer pool, and keeping the stock part in a box is worth more than most people assume. And warranty: depending on where you live and what you changed, a modification can affect a warranty claim on a related system.
None of that means avoid these parts. Some of the best-value modifications on any bike are "requires modification" jobs, and riders do them successfully every day. The point is that the label is a signal to plan, not a footnote to skim.
Before you buy something marked this way, three questions are worth answering. What exactly has to be modified, in specific terms — which panel, which wire, which bracket. Is it reversible. And are you doing the work, or paying someone who has done it before on this model.
If you cannot answer the first one, that is the thing to resolve before spending anything. The forums here are a reasonable place to ask, because somebody with your exact bike has usually already found out.
This site can tell you that modification is required. It cannot tell you whether it is worth it to you — that depends on your bike, your tools, your budget, and how much you care about going back to stock later.