Ouch. I collect Walthers mainly. I just started really collecting them not so long ago, so you know what they say about a little knowledge?
Collecting Lugers is probably the single biggest risk you can take in buying guns. They are all expensive. Even mistmatched "shooters" start at about $700. Most models are $1500-4000. Even the holsters can be worth up to $800 by themselves. The "take down tools" are expensive. Additionally, because they are so valuable, there is a ton of fraud. People "mock up"guns. I even had a guy tell me that some industrious crooks are stamping out magazine numbers to enhance the values on antique presses.
There are lots of variations, but generally Lugers can be divided into the following catagories:
Pre WW I
WW I
Weimar
Nazi/WW II
The most valuable are certain pre-War and certain Nazi marked ones.
Condition is everything. The better the finish and bore, the more valuable the pistol. Matching serial numbers are a must, except most magazines won't be numbered to the guns (when you do find this they command a premium). Good condition holsters and take down tools improve value.
Later I will add some links. Tired now.
Then it breaks down by company i.e. Mauser, DWM, and by country manufactured i.e. Germany, Swiss, Finnish, Caliber, barrel length, sights, stocks(not grips). Then above all else condition, condition, condition. Tools and all acessories. Besides the 1911A1, the Luger is probably the most recognizable gun in the world, and the most collected of any firearm.
Go here and have a look www.simpsonltd.com Luger heaven.