I would look for pay dirt that is similar to the type of dirt you plan on panning.
if you want to practice panning for gold or to test your Prospecting Equipment, pans, sluices it is better to buy 1 gram of Gold, in various sized mesh (4,6,8,12,16 etc) for about $44 / gram. You can add this gold to your own concentrates, sand, gravel, black sand blond sand, and know how much gold you should recover. This way you are paying for gold and getting gold. You are also keeping your Shipping costs very low. Why pay for 5 lbs of dirt + the Shipping costs for 5 lbs of dirt.
A good site is http://goldpaydirtreviews.com/
there you can see what the dirt looks like from various companies.
Like Bill said "Most of all, have fun"