Welcome Betty.
I don't know what others would say, but I would have to expand what you have been told just a bit.
Firstly, I have seen greater price differences than that. For e.g. a seller may sell stamps in only superb condition, or stamps certified superb. In this case I have seen a lot of sellers use this as "an excuse" to price them sometimes ridiculously high compared to what they should be. This is where I see those differences mostly. Taking advantage of stamp condition is unfortunately common for some sellers. And the cost to certify a stamp is not as much as the hundreds of dollars that are added to the price. When you have wealthy buyers and you add the old saying "there's a sucker born every minute", warm, stir well and serve, these sellers do sell their stamps.
Secondly, the only way to use sites like
ebay to determine the actual value of a stamp is to use several auction sites and, keeping in mind the above, look at the closed auctions at these sites and see what the stamp actually sold for at the different stamp conditions you see. You will then not only get a feel for what the range is for a particular stamp, but you will be able to spot those higher sold prices easier for there will be fewer of them in the closed auctions than in the active auctions.
It is a bit of work, but we have to be careful during the thrill of the hunt!
Hope this helps.
-IBFS