Why does by-the-glass wine cost more than by the bottle?

Why is there rarely a fair comparison between the price of wine by-the-glass compared to a bottle of the same wine? Especially when (at the lousy pour size of 100 millilitres) the restaurant is getting at least seven glasses per bottle? I accept that (as for most things) it's cheaper to buy in "bulk" – but I don't accept the oft-given argument that some wine may go unsold if sold by the glass when there's always only a limited range available. Are we by-the-glass consumers being ripped off ?

In the pages immediately preceding the epic hangover scene in Kingsley Amis's immortal Lucky Jim, the novel's hero, who is staying the night at his boss's house, describes a bottle from which his thrifty host has poured "the smallest drink he'd ever seriously been offered". It's a phrase that sometimes returns to me when I order wine by the glass and receive what might best be described as a puddle – which is what 100-odd millilitres looks like in a balloon-like burgundy glass – and am paying north of $15 for the privilege.

But you know what? It's not something I see all that often. When I do, it's almost invariably at the stratospheric end of the restaurant market, at the kind of place where opening the wine list makes me gasp - with delight at the selection and dismay at the prices.

Perhaps you patronise such places more often than I do; in my dining-out experience, by-the-glass pours of still table wine are generally closer to 150 millilitres. So, excepting champagne and dessert wines (often sold as smaller pours), that's the equivalent of five glasses to a standard 750-millilitre bottle.

Having said that, you're correct in observing that by-the-glass wine is often pricier than by the bottle but I am not convinced this amounts to a rip-off. Even when the by-the-glass range is limited, not all open bottles are emptied every night. They will be re-capped and perhaps treated with preservative gas at the end of the night's service but, depending on the wine and the level on the bottle, that may not be enough to maintain freshness. On subsequent days, conscientious sommeliers check previously opened wines before pouring them for customers; a fair bit does get tipped down the sink.