Like anything, I think it depends on the machine. I am willing to pay quite a bit more for a machine that I have been looking for for ages, if it is in great condition, with case also in good condition etc. ready to type.
Could be $100 in regular used, condition and $450 in top notch condition??
Depends on how much you want/love it I guess...

Marcham93 wrote:

How much extra would you pay for a professionally serviced machine? ...So, how much would you pay extra for a machine that has been cleaned out, oiled, thoroughly tested, etc.?

First of all, welcome to the forum. 

There are a number of important variables that you would need to verify first before even entering a discussion about cost. First, was the typewriter actually professionally serviced? Nine times out of ten such claims are made by those who just blew dirt out of the machine and don't otherwise know anything about typewriters. Anyone who is selling a typewriter at a premium should be asked about who exactly serviced it, and what that person's training/experience with typewriters is.

The second question is to get a list that details exactly what services were performed. Just claiming that a machine was serviced is too ambiguous; there's a big difference between someone just giving a machine a cursory cleaning and one that has been properly restored and has had parts such as the platen resurfaced. 

As others have said, it depends.  My first typewriter falls into this category, I think.  I bought it online. later heard that this person charged high prices.  However, the machine arrived to me safely and I have not had to have it serviced for anything since then.  I bought the machine in question because it reminded me of the one that my mom had used to type my term papers when I was in school.  She no longer had that one or else I would have had it serviced.    

It would depend on how much work you would want done, and/or how much needed to be done--meaning what needs to be done right away.  Offhand, knowing IBMs, the first thing would be to brush the rubber dust out of the power cams from down below (check them out and see if they don't have black all over them).  The power roll, if you can get it recovered from J. J. Short, would be good too, and add to the life of the typewriter.  Also, the impression control on the right side of the keyboard should be set between 1 and 3, and the multiple copy control should be set to the letter A.  Get the typewriter to type a decently dark page within these parameters and you'll have a very nicely performing typewriter.  This, and a light oiling of the carriage rails, and a VERY judicious wiping of an oiled cloth over the segment slots (wipe off the excess--you DON'T want ANY oil getting onto the power roller.  If some does, wipe it off very carefully and make sure none remains on it.  This is ONE part you want dry.)  Anyway, these are things I intend to do with my '56 model real soon.  It uses both carbon and fabric ribbons--though you have to unload one to use the other.  I'd like to get it to typing again too!!