Seriously, how much a puppy costs depends greatly on the area you live in. Differentials can be huge.
Whatever the going rate in your area, I would strongly urge you to buy your puppy from a reputable breeder, preferably one that shows their dogs (you want a boxer that looks and behaves as a boxer should), but
definitely from one who does proper health screening for gentically inheritable disorders. In the case of the boxer, the parents must be screened for hip dysplasia and for the heart conditions aortic stenosis and boxer cardiomyopathy. Those, sadly, are conditions common to the breed and will impact
severely on the quality of life of your pup should it inherit them.
Here is a link to an extremely useful site that lists genetic conditions common to many breeds (so you can also read up on what labs and rotties are prone to and should be screened for - you need to scroll down to dog breeds and select from there):
http://www.upei.ca/~cidd/intro.htm
And here is a link to the American Boxer Club's recommendations for health screening of breeding boxers:
http://americanboxerclub.org/health-screening.html
Any good breeder should be doing all of these tests, should be able to provide you with hard copies of the (satisfactory) results for both parents, and should be prepared to offer you a minimum of a year's guarantee against genetic health problems (preferably 2 or 3 years).