You'll pretty much want to go with one of your favorite players. I voted for
Steven Jackson, because he may be the most popular Ram to those outside the Rams fan-base right now, either him or Holt I believe.
As for price, your base price will be something like $74.99. Most of the time you can find a store (Hibbet, for example) that will have some for 50-60 bucks.
Of course ebay has a pretty nice selection, but you have to watch out for fake jerseys, or jerseys not made by Reebok (Rams nike jerseys are a lot cheaper, but they are not the true replica jerseys).