An antique exterior door might have beveled glass or brass doorknobs. You can choose to purchase one of the very expensive real antiques, buy something that has an antique-look or pick up something that has been salvaged from an old home.
Prices vary greatly. A simple set of brass door knobs can cost hundreds of dollars. A salvaged door would probably need new hardware like that.
There are other things to think about, too. If you decide to refinish a salvaged door, you will need to sand off any old paint or stain. Before that can be done, you will need to remove any existing hardware.
When you remove the hardware, you will want to inspect it for damage. The question to ask is this. Can the hardware be reused?
If it is not in very good condition and has no aesthetic value, it is best to replace it. A doors look is obviously important, as it is the first thing that your guests see when they drive up or walk towards your home. But, functionality is the most important consideration here.
If you find an antique exterior door that is not truly functional, but you love the look of it, consider using it as an interior door. Adjustments will likely need to be made, because interior doors are, in general, smaller than entrance doors. But, with just a little handiwork, you can keep your work of art without worrying that the wind will blow it down or a burglar will break through the lead glass.
Some of the antiques on the market cost nearly $20,000. That is a bit extreme for most homeowners. The value might be in the wood. For example, French walnut is quite valuable. But, it sometimes has to do with where the door came from.
If you decide to pay the price for a true antique exterior door that came from some famous European spot, try to make sure that there is some verifiability to the dealers claim. Remember, you do not really know where it came from. You were not present when it was removed.
If you decide to order doors for your home over the internet, be sure to measure the size and find out if returns are accepted. Unless shipping is free, the shipping charge could be more than you had expected. The packages are large and awkward. So, they cost more.
Personally, I prefer something new that just looks like an antique door. But, not everyone would agree with me.