The cost is often higher because of the labor involved in making them and the cost of the gold. The thinner the chain (if gold), the cheaper the necklace, and too thin of a chain can be problematic (it can easily break).
The cost when dealing with freshwater pearls is especially noticeable because freshwater pearls are fairly inexpensive.
It is much less of an issue with Tahitian, South Sea, or larger Akoya pearls. The tin-cup will nearly always be less expensive than an entire strand.
The pearl quality typically is equal to a similarly graded strand, if ordered from the same retailer.
Here's a tin-cup we recently made with B quality South Sea pearls (using the A-D Scale). You can imagine how much gold is involved in the chain in this necklace... The pearls are 12-14mm.
We'll be soon adding an entire section to our website just for tin-cup necklaces with nearly every type and quality of pearl we sell.