Understand that your local coin dealer is there to provide numismatists with rare coins. This is his business all year, and you are his nuisance only when the economy sucks. Rarity can be rarity, as in that mint minted few that year, and it can be something special (like the last 1803 P half in pristine uncirculated condition, or a mistake that escaped inspection when it was minted). This isn't what you're after. As long as you understand this about the proprietor, you'll get along with him or her better.
U.S. coins from the dime up minted in or before 1964 have silver. Few minted in or after 1965 have any silver at all, and none are 90% silver like the old ones. I have been managing to get old U.S. silver coins at FRN 12.25-13.00 to the $1. So, a silver dollar costs up to thirteen, dimes up to 1.30 or I don't buy. Likewise, I'll gladly pay three bucks for a quarter. The alloy is identical (some variance over time, but none in a particular year's dimes, quarters, halves or dollars). The weights are proportional. Pay half as much for halves as you pay for dollars and etc.
The more you buy at once the better a price you get.
If the dealer paid too much, he or she will either offer to sell to you for too much or will leave that stock in the safe. You can't reasonably expect them to take a loss just to sell to you.
Know the spot price before you go, but don't expect to pay only that. They have bricks and mortar to maintain, electric bills to pay...
U.S. gold came in $20 double eagles, $10 eagles, $5 gold pieces and (briefly) some smaller denominations years and years ago. The smaller ones were generally deemed too easy to lose (and a dollar was a hell of a lot of money at the time). Double-check me, but I think the double eagle is an ounce, and weights are proportional to face value here as well.
That help?