[brewing]

Popular usually equals expensive.

The must underrated popular player as far as prices is Stan Musial. His numbers are amazing, and his cards are reasonably priced. There is a limited number of years he had a card during his playing years.

Personally, the first player set I completed was Al Kaline. Very underrated, his numbers are on par with Clemente, but his cards sell for 1/2 of his.

Besides F. Robinson who is very underrated among collectors, the following are undervalued in my opinion. All are members of the 500 HR club too.
Eddie Matthews (only his hi# rookie is expensive)
Mike Schmidt (greatest 3B ever with all cards besides rookie available under $20)
Ernie Banks
Willie mcCovey