There are a few thing you need to consider. Age, overall condition, popularity. You can sort most of that out looking on eBay. If you find sold listing for very similarly sized, and condition Joseph Rogers Razors then figure out what they average, that’s pretty much the value. If you are looking at it for insurance purposes, and are trying to get a decent value on a recent gift or inheritance, then go with the highest price of same or extremely similar razor. Some brands sell higher than others for similar products, so keep the brand size condition and overall shape as close as possible. It is difficult if you don’t understand much about sizes, grinds and scale materials.