How much are old 78s worth?

The value or price paid for any old record is market driven, and varies. eBay (searching completed auctions) is a good indicator of current trends. Sometimes, there is no rhyme or reason in terms of what someone is willing to pay.

To find out information about a specific 78 rpm record--the recording date, artist, etc.-- go to one of two comprehensive online discographies:

http://adp.library.ucsb.edu/ -- which contains a discography of most Victor recordings...

http://www.78discography.com/ -- which provides information about Victor, Columbia, Decca, Brunswick and other brands.

Another option is to do a web search for the title, catalog number (which appears on the label), label name, artist, etc. Oftentimes, references to specific records appear on forums, in sales lists, etc. (This can be especially useful when researching an LP, or later production 45 rpm record.)

In addition to rarity, condition is a big factor.
Here's a basic grading guide -- http://www.jazzhound.net/record-grading.html

This article may also be helpful. Detailed and comprehensive -
http://www.78rpmcommunity.com/beta/articles/8/valuing-78rpms