There might be a blue book on skid steers and if there is it probably costs $500 and is not worth buying. Prices vary by regions due to the economy and other factors. A cab and heat machine is not as valuable down south, machines near Detroit are not going to get a premium comparable to growing cities like Omaha and Huntsville (may not be growing anymore). I always use machinerytrader.com and rockanddirt.com they will give you an estimate to what people are asking for machines. Machinerytrader also shows you previous auction prices that can be helpful in determining value.
One could take auction prices and retail asking prices for a machine and use those numbers to find a range for a machine. In theory if the machine you are looking at would fall in the range then it would be reasonably priced. There are so many variables although that determine price. Engine hours, tires, wheels, bucket and accessories, auxiliary hydraulics, self leveling loaders, ride control loaders, glow plugs, block heater, previous services, paint, seat, hydraulic system (hoses,pumps,cylinders),drive motors, dents, battery, and so on.
The rental companies usually have good prices. The RSC near to me has been selling off some of its equipment at fairly decent prices. I would think that some of the small rental companies might sell for an even better price at this time. I would think that not as many people are renting machines due to the economy. When you buy a previous rental machine you are getting what you pay for though. Who knows how the renters treated the machine or what attachments were used on the machine. A concrete breaker can wear pins and hydraulic pumps quicker than a machine that was just used for dirt. Those machines could have gone 150 hours without being greased. The positive is that they most likely were serviced every 250 hours so one might think that the engine and fluids would be good.