I haven’t checked prices in a while, but decent used anvils were running around $2 a pound (and up), several years ago.
If the anvil is too light for what you are doing, you are going to have problems, but having ‘too big’ an anvil is mainly an issue that comes up when you have to move it. Something around 100 pounds (of decent quality and condition) is unlikely to disappoint you (not too heavy to move, and not too light for a starting anvil).
Years ago, somebody told me that it’s a good idea to keep a large (1/2 inch to 3/4 inch, or so) ball bearing in your pocket, if you are looking at used anvils. A quick way to tell if an anvil is ‘bad’, is to lightly tap various spots of the workface with a hammer and listen. If it has a consistent ‘ting, ting, ting’ sound all across the workface, then the hardened workface has not separated from the softer core of the anvil. Cast anvils are a little different, in that only the high end of the cast anvils (cast anvils are generally considered the low end of anvil quality) have a hardened steel workface attached (during the casting process) to the softer core. The rest of the cast anvils are cast as a single piece, then the workface is hardened (after polishing, to some degree) through some form of heat treating.
Some people tend to get bent out of shape if you start tapping on their anvil with a hammer (and hammers are harder to carry around than a ball bearing), and this is where the ball bearing comes into play. Hold the ball bearing 10 inches or so above the workface, then drop it. In addition to listening for the ‘ting’ on impact with the workface, watch how high the ball bearing bounces. If it has a good quality workface, the ball bearing will bounce back almost to the height you dropped it from. Softer workfaces (which are not limited to poor/cheap manufacturing, but can also be an indication that a good quality anvil was ruined by being in a building that burned down, destroying the heat treat) will result in the ball bearing bouncing back to a lower height. The lower the bounce, the more of the impact energy the anvil is absorbing (soaking it up in it’s core). You don’t want the anvil absorbing the impact like that, as it is taking energy that should be going to forming the piece you are working on (you will wear yourself out, using an anvil like that).
You can probably assume that most new cast anvils are worthless for bladesmithing, or any other type of smithing. I have a 24 pound cast anvil that I bought new, a couple decades ago, and it sits on my toolbox at work. The face is soft, but it does OK for cold working aluminum. If I had intended to use it for hot work with steel, I probably would have destroyed it in a matter of days (anything more than a light hit with a 12 ounce hammer leaves a dent in the workface). The example Robert provided is likely a decent anvil, even though it is cast, but the price reflects that quality.