Unless you have some rare or very desirable stuff, my experience is that 78s in very good condition typically sell in the $5-$10 range.
Many of the problems with accurately pricing very old records, are similar to pricing newer records: Original issue or re-issue; Condition; completeness of original packaging. A tanking economy doesn't help either.
There are many price guides. It would be best to look at a number of them, and get an overall feel of what the price guide writers think the market looks like. Then, subtract 25%.
Perhaps the biggest bounding element is simply whatever copies are already available for sale on line.