Loupes- Introduction

The loupe is an indispensable tool for the micromineral collector, geologist, or mineralogist. Many micromineral specimens are viewable with nothing more than a good loupe; even in those instances where a loupe is not quite powerful enough to study the tiny crystals in a rock, it can help the collector decide whether or not to take a specimen home for microscopic study.
The ideal magnification for a loupe is 10x (ten power). Decent 15x and 20x loupes are more expensive and smaller in field of view than the 10x ones. Usually you have to put the 20x loupe (and your face) so close to the specimen that it blocks most of the light.  It's self-defeating.  Lens quality also makes a much more noticeable difference at these higher magnifications.
From here on in I'll be referring mostly to loupes of the 10x variety, unless otherwise stated.
First of all, not all loupes are good. Without going beyond the scope of this website, I'll just say that the simple, single-element lens used in "no-name" loupes has inherent problems with image distortion, blurriness, or distracting red & blue halos. Sometimes you get lucky and end up with a single lens that gives a good image- but you can't rely on that.
To compensate for optical aberrations and make consistently good lenses, manufacturers have resorted to multiple lens elements in one loupe (usually three, called a "triplet"), or sometimes a single lens which is thick and has a special groove around the circumference to give a similar corrective effect (this special lens is called a "Coddington").

Varying Prices & Qualities

Inexpensive, single-lens loupes can be had for as little as US $3 to $5. Some of these are essentially junk, while others are pretty good. It's luck of the draw- but low quality predominates. The majority are unmarked, so it's difficult to tell a good one from a piece of junk without actually peering through the lens.
The classic, 3-lensed triplet loupes range from as little as US $20 for the nice Belomo, to US $50-$75 for the [also excellent] brand-name Hastings Triplets by Bausch & Lomb. Other manufacturers such as Nikon produce great loupes, but the prices are $75 and up.  I can think of at least one make that's in the $300 range.
In between the previous two types is the Coddington. This is actually closer in price and quality to the triplet, and for many applications will provide nearly equal results. Bausch & Lomb even manufactures one that has built-in illumination... very useful.
Less frequently encountered is the "doublet", so called because it uses two lenses instead of three. Doublets are sometimes offered by optical companies as "affordable alternatives" to their triplet models. However, if you can get your hands on a Belomo, purchase one of those in preference. Some doublets actually have a Coddington lens plus a regular lens... I haven't tried one of these, but I'd expect the image quality to be very good.
There are also loupes with multiple but independent lenses. What this means is that you can choose one lens of 5x, 7x, 9x, etc... or a combination of 2 or 3 lenses to give yet higher magnifications. The individual lenses can be handy, but using them together to get higher magnification is never quite as good as using an actual triplet loupe.