To look up the price of similar razors on eBay is a fairly simple process.
The most important thing is establishing the search terms... If you are looking for a common brand name, such as "Wade and Butcher" use that term for your eBay search.
If you are looking for generic vintage 5/8 razors, "vintage straight razors" is probably your best search - and you will need to weed out some results.
Once you have established your search criteria, do a search, and then click on "completed listings" in the "Show Only" box. This will show you completed results for the razor you are searching for.
You should be able to go through the completed auctions and find sales results for razors similar to yours.
Remember to compare the condition of the razor you have for sale to the prices - if you have a rusty 8/8 Wade and Butcher with chips in the edge, it won't be worth nearly as much as a clean vintage 8/8 Wade and Butcher in good condition.
If you have a generic Solingen, Sheffield, or Eksiltuna razor, be sure to focus your search on those.
For generic hardware store razors, you may have to just do a search for "vintage straight razor" and weed through the results.
To get the best idea of a price range, find 15 or 20 razors similar to yours and look for a high-end, low-end, and average price. That should give you a rough idea of what your razor is "worth" on the current market.