Anvils used to go for $1-2 /pound. They tend to be more expensive these days because collectors like them. So you can expect to pay as much as $3-6 / pound for a good anvil.

What's a good anvil? One that has a good rebound. Bring a ball bearing, drop it on the anvil surface from about 18" and you should be able to easily catch it when it bounces. If it doesn't bounce up nicely, you'll end up putting a *lot* more work into moving your metal. 

Also: check the edges. Watch out for chipped edges. You can work with them, but they reduce the value of the anvil.

Check the face of the anvil for cracks and deep grooves. Those will show up in your work like stamps. You can work around them but they reduce the value.

Of course, an antique dealer really doesn't have to care about whether the anvil is in working condition; they can sell a busted anvil to someone with money to burn as a lawn ornament for the same amount or more.

I really like the German-style anvils (also known as the double-horn anvils).  They have a normal horn on one end and a tapered, square horn on the other.  Very useful. 

You'll want an anvil that weighs about 150 pounds (or more) to get a reasonable amount of work out of it.  Smaller anvils can be useful for packing in your luggage and sweetly replying "yes, of course" when someone notes how heavy your bag is and wonders aloud if you've packed an anvil in it. 

Not that I've ever done that.