In many Topps sets the cards can vary by series as either gray or white backs, and in some, 1959 and 1960 come to mind, even a single series can have mixed backs. For those two years the price differential for the different backs is not too significant. But for the first 50 cards of the 1953 set, and cards 131 to 190 of the 52 set, the differential for the gray backs is significant.
These cards are sometimes referred to as "Canadians" because it was speculated that they were produced and distributed for Topps in Canada . I believe there is some hobby dispute about that, but not about the fact these gray backs are scarce compared to the regular cards. The fronts of these cards also tend to be darker and less focused, but apparently not always so. Top condition 52 gray backs in the 131-190 range often go for more money than the high series cards.
If you search for Topps 1952 gray backs and or Canadians on ebay you will find several examples, several offered by 707 Sportscards. Remember there are other 52 gray backs in other series in the set. You should look for those gray back cards numbered 131-190
I only have 11 of the 52 gray backs but there are a couple of collectors here with almost or completed sets. Huggins and Scott recently offered what it called a Topps 1952 Super set which included a full set of the grays and other variant cards not listed for the "normal" Topps 1952 master set
http://www.sportscollectorsdigest.co...th-two-mantles
http://www.net54baseball.com/showthread.php?t=172391
http://www.sportscollectorsdaily.com...scott-auction/
http://www.ebay.com/itm/1952-Topps-G...item4854e530cd