There is only one specie of diamondback terrapin (Malaclemys terrapin). There are 7 recognized geographic variations (subspecies). The 7 subspecies vary in pattern, coloration and other morphological characteristics. Even within a single clutch of the same subspecies you will see variation.
The mangrove diamondback terrapin (Malaclemys terrapin rhizophorarum) is by far the rarest of the bunch.
Many of the subspecies can be found for $20-$50 as hatchlings. However, nowadays it's not uncommon to find money-hungry slave traders selling them for $75 and up. The ornate diamondback (M. t. macrospilota) and mangrove terrapin hatchlings are sold for $100 these days. Also, some people call a pattern variation of northern diamondbacks (M. terrapin terrapin) "ornate" and command extra money, for whatever rediculus reason.
Check out Jonathan's Diamondback Terrapin World on the WWW. There is a lot of good info and pictures of the different subspecies on his site.