In the last two years (2007, 2008) commercial calf prices (not registered or show stock) have been lower than average, which seems to relate to very high feed costs. Day old dairy calves have sold for $5, for instance. That is the purchase price; the price to wean is higher. See the links for the current costs on weaning a calf and how to locate one of these calves in your area.
Average prices (2008, west coast of the USA) for 400-600lb calves beef calf is $1/lb. For a dairy steer, $.75-80. cattle prices are widely reported as a commodity item. Check the wall street journal or barrons for current prices. Local auctions will be higher or lower randomly, but usually within 15%.
There are two main varieties of calf typically available; beef calves and dairy calves.
Beef calves will put on weight faster and typically result in more pounds of beef for fewer pounds of feed. If you are paying for your feed, it's usually worth the higher cost for a beef calf.
Dairy calves are a byproduct of the dairy industry. In order to milk a cow, she has to be pregnant and half of the resulting calves are male, which aren't used in a dairying operation. these surplus calves will be lower cost, but typically require 2 to 3 months of work before you can turn them out on pasture. So the lower initial cost is often offset by the cost of milk replacer and labor. Some dairy calves are half beef, half dairy. Typically these will be a little more efficient than a pure dairy in growing to slaughter size. If you have pasture available or a low-cost source of feed these may be the lowest cost option.
The health of the animal is more important than saving a dime a pound on the purchase price. If this is your first animal you should consider paying a little more to a farmer you trust to help you get a healthy animal.