Digitalbrain: there is no simple answer. I agree with Webwork that online tools for assessment are for entertainment purposes only.
There are too many variables. Are you selling to an end user or a reseller? are you selling a group of related domain names or selling one by one? do you have traffic and revenue (as Webwork alluded to)?
You sometimes get some guidance from prior sales of related domain names but even then you dont know the variables that went into that sale price.
There are some companies that offer appraisal services. Again I do not find these useful as they often give you a very wide range for valuation.
The probably not so helpful answer is that the price is what the market will bear. Ultimately that comes down to whether you sell through an auction site or online brokerage with your names buried and competing with many others for the buyers money or if you spend time hunting down prospective end users. The motivation and desire of the prospective buyer also plays a part.
There is nothing to beat experience selling domains to help you value your domains and even then i have been surprised at how much higher some buyers will pay for certain domains.