If it is for sale or resale I would point out that the retail price will ultimately be determined by the buyer. You will get the highest price by selling it yourself to the end user. If you plan on selling it through a dealer, he will generally give you, as a maximum, one-half of what he thinks he can get for it. If it is for insurance purposes you must go to a bonded appraiser or auction house, preferably one with experience in historic textiles. They will provide an estimate in writing.
Flags are generally worth only their replacement cost.
The same flag used by the USMC on Iwo Jima cannot be had for any amount of money. It is extrinsic value that makes flags more desirable. So it is incumbent on the owner of the flag to document the chain of possession, the flag's supposed use, and any photographic evidence, otherwise it is all oral and therefore of no additional value. Every hamlet in the Third Reich had hundreds of these so they are not rare. Size is a factor as the larger flags are worth less because they present a storage and display problem for most collectors.