You can go to PSA's website
www.psacard.com or SGC's website
www.sgccard.com to view the grading fees. I believe they both have a $15 per card fee for cards valued at less than $500. There's also the cost for shipping and insurance both ways. If you think you can turn enough of a profit selling them graded as opposed to raw it may be worthwhile. I'm not sure what dollar amount you mean by "postwar cards without a high dollar value", but you would probably have to be able to make an extra $20 per card just to break even.