The value of your coin is relative. It depends on your intent. What do you want to do? Sell Nanna's coins? Obtain dividends on an investment? You need money yesterday? Let's call the price you would expect to see in a coin shop is the maximum value of a coin. At an auction, you would expect to pay less for the same coin. If the coin is certified, it can be sold for more in any venue because you are relying more on concensus opinion rather than any individual's opinion.
The guy in the coin shop has to make a living. If he sells a coin at 100% value, he has paid somebody 10% for the coin. I would never do that. But I don't need money yesterday.
Before I forget, don't clean a coin. Don't even touch it. No. Do not pick it up with tweezers. The coins have been touched lots? Oh well. Just not as valuable as a never touched coin. Take a shiny new penny. Put your thumb on it and then put it away for a year. You'll see your thumb print on it. Extrapolate.
Coin 'guides' typically give you the price your coin would fetch from a dealer. You can tell because the book is thin. For each coin year, there is one price. It doesn't take grade into consideration. That's the coin dealer's best friend. 'See? It's right here. It's worth a dollar fifty.' It's all relative.
The thicker coin books are good because they are informative in terms of history and provide an important 'provenance'. But they are only published once a year and so values, although taking grade into consideration, just tell you what coins are worth on an annual basis. Because of the variability of a number of factos such as the current global price of silver, coin values can fluctuate on a weekly basis. One week my collection value jumped not because of rarity of the coins but because the price of silver went up.
There are a few factors involved in determining the value of your coin(s).
1) The real rarity of the coin. How many were minted but how many are estimated to remain available. 1921. They produced all the denominations. Gave out sets as souvenirs to visitors to the mint. Then they melted all the coins they made, leaving only the visitor's sets. Very few relative to the initial mintage. An uncirculated 1921 fifty cent piece is worth about $500,000.00.
2) Grade of the coin(s). Some coins survive even hundreds of years without ever having been touched by human hands. Incredible forsight I think for somebody to say a hundred and fifty years ago 'Let's not touch this coin. It'll be worth lots someday.' I have a 1948 Upper Canada Penny which not only has never been touched but it is as shiny as a new penny out of your pocket (yes I know they're not made anymore. Just painting you a visual). You can't touch a coin because the acid in your sweat will corrode the coin. Take a shiny penny and press your thumb into it. Put it aside and in a year you'll see your print. BTW as soon as you clean a coin, even lightly, it's largely worthless. Don't try to be tricky. It's not possible.
3) Esthetic qualities. Difficult to put prices on a lot of my coins because of this aspect. You can't get those aspects certified because beauty is relative (see title page). Silver corrodes only sometimes into brilliant colours like blue and red. Those aspects typically command a premium because of the additional rarity component. Luster is another such factor. Some coins glisten like the Northern Lights.
4) Coin Material. The value of my collection increased substantially one year because the price of silver went up substantially.
All of these factors combine to result in the final 'value' of a coin.