MG,
"Mr. Sabato" is my father
... please call me John.
I responded to your questions about this piece because I enjoy examining photographs of Lugers, and I have studied them for several decades... I am not an author on Lugers, nor do I consider myself an "expert" -- but I am very knowledgeable about much Luger history and manufacturing processes and gunsmithing techniques...because I have done a lot of reading of Luger books written by people that are considered experts.
I am also very capable with a computer (make my living with one) and digital photographic techniques to improve detail... this makes looking at auction photographs an interesting pasttime for me...
Most auctioneers will not tell you their reserve price...if they did, it would serve them no purpose... if they don't get their reserve price, they just re-list the item and try again. If they hold out long enough, they are gambling that someone will come up with the money.
The value of this or any other gun depends on the condition of the gun and many other factors...originality of the parts, finish, consequential damage from handling, the rarity of the model and many other things, but especially what is the gun worth to YOU?
I cannot guide you on what value this gun has for you... If this gun were to complete some kind of collection for a person, it would have more value that for someone who just wanted a long barreled Luger to shoot...
You have already stated that the rear sight is not matching and I have explained that the front toggle appears to be non-matching... that makes this a parts gun. It is not a collectable gun, but to someone whose grandfather used one in World War one it might be exactly what they are looking for to honor their memory and they would pay a bunch more than someone who just wants to shoot one.
I bought both of the Lugers that I own over two decades ago for less than a total of $200. That was a LOT of money to me back then (almost half a month's pay), but I wanted those Lugers really bad.
You will have to decide what the gun is worth to you before you bid. Perhaps you need to do a lot more research before you are ready to buy an auction gun. Like for the next few months, you should watch every auction of an artillery and catalog the sales price and the condition of each one... then come up with your own statistics on value and look for the next one that meets your needs... AND your wallet!
In the meantime, you can offer a price of $25.00 above the current bid until you reach the limit of your budget for your "shooter" -- There is no magic rule for buying old guns... buy the best you can afford, and when you can, trade up to improve your collection. If you get outbid, then I am afraid it wasn't your turn...maybe next time.
Good Luck.