I can get about $350.00 for that size in my market. You make your money on the install, which is your rate x how much time you think it will take. If you've got the only install crew in town, you can ask a higher price, but most of us are dealing with dozens of competitors which generally keeps the price somewhat stabilized.
Several contributors have commented on the advertising value of the sign. While this is true, OOH plant owners know the going wholesale rate for covers. Many even print their own. Small billboard owners will use your service, but if you mark up too high they will go elsewhere. That "advertising value" is determined by the board location and the strength of the message. If you do not own the location, you cannot expect to cut into the owner's profits just because you know the advertising value of the location. Likewise, if you are not creating the message, you can not reasonably expect the client to pay more because his art is good. He has already paid his agency or designer to create that message.
Now if you are creating the message, and you pay your designer to come up with a killer design, now you can charge for some of the advertising value of that message. It's a fact that most small plant operators and individual board owners are fairly clueless when it comes to effective outdoor advertising. A good message that is clearly visible and easy to read, one which supports the branding and marketing objectives of the advertiser, can increase the value of that board multiple times. If you have the knowledge and design chops to produce effective billboard advertising (and can convince the client of its value), you can charge accordingly.