The best way to know if it's "too good to be true" for manufacturers like DJI and GoPro or similar is to look at the prices at a variety of known legitimate resellers. If the price is the same everywhere then you know that the manufacturer tightly controls what the products can be sold for. Some companies won't let you distribute a particular product unless you agree to sell it for a certain price. Both DJI and GoPro appear to adhere to this kind of control.
So, if you see the product listed for less, even 10% less, you should exercise caution, because somehow they've broken that tight control. It's possible that something is not up to par.