Hi,
Without more info or pics it is difficult to provide indications of value. But let's try.
Scenario #1. Your Dad soaked stamps from envelopes in the mail and put them in an album or a box or folder. These will probably be worth less than a penny per stamp and probably will be rejected for an appraisal.
Scenario #2. Your Dad accumulated multiple closets of stamps in boxes, bags, etc. (hoarded?). These will probably be worth less than a penny per stamp and will probably be rejected for an appraisal. However, there are collectors (and some dealers) who will make you a relatively small offer (equivalent to $50-$100 per 20,000-50,000 stamps, give or take) based on weight or volume.
Scenario #3. Your Dad had a strong knowledge of which stamps were valuable (or
inherited a quality collection from his Mom or Dad), and spent a fair to substantial sum of money putting together a good looking collection. An appraisal is warranted and may yield value ($).
Obviously there are other scenarios, but the above three are very common. Hopefully, this synopsis saves you some time and/or money.

Radical Idea #1. You (and/or your Mom) take your Dad's collection and expand on it while having hours of fun, learning and excitement (well, within reason of course

) in your new hobby of stamp collecting.
Best of luck,
Greg