This type of question is posted all the time around here and I will give you the same answer I always give. Domains/websites are just like any other business. Value them the same way you would a restaurant, grocery store, or apartment buildings.
In other words, how much money is the business currently making. All industries have a 'normal' multiplier. It basically boils down to one simple equation, how much are you willing to pay for that cash flow? Cause in the end, that is what you are doing. Paying for the current cash flow. If it isn't making any money, it isn't worth anything. If it is making money, conduct due diligence, and apply the normal multiplier. Normally, you will end up with a range of values. If the business has upside, expect to pay on the high side of the multiplier. If there is not an upside, expect to pay on the low side of the multiplier.
If it is above your range, simply walk away. And never, ever pay for potential. Potential works both ways. It can potentially go broke or make take off like a shooting star, based on your work. The old owners shouldn't be paid for your hard work.