Hi rubblekrats, and welcome to the Forum.
If you don't want to invest in the latest stamp pricing catalogs,
many of us refer to online resources as ikeyPikey suggested, or we buy used stamp catalogs available on
Amazon and
ebay. Since I assume your collection is worldwide, if you buy the Scott Catalogs there are a number of books which does become expensive. I find libraries sell their stamp catalogs when they replace them with the newest yearly version. Buying the 2014 Scott catalog (for only US stamps) was far cheaper than buying the new one. But having my own catalog at home allows me to work on my collection any day or time.
You can post good scans here of some of the oldest stamps and folks in this Community will be better able to help provide some guidance. Enjoy!