First, the problem with sitting on your nuts: you can mount without your full weight on the saddle, then as you sit down, slide yourself slightly backwards and the seat slightly forwards. This pulls the vulnerable parts out of the way. or you can do what I do, and swing them over your shoulder - this is easier on a warm day.
Now the difference(s) between a cheap unicycle and an expensive one…
Yes, there is almost certainly an element of fashion, and if you pay twice as much, you might get something that isn’t twice as good - but it will be better. This is the same with any piece of sports equipment, or, indeed, most consumer goods.
Working from the ground up:
Tyre (US = tire). At UK proces, you can buy a knobbly 24 inch tyre for about 4 Pounds (approx US$6). You can also pay 30 Pounds or more. The better tyre will be more durable, have a bigger profile, better designed grip, and so on. Probably more important on an off roader or trials uni than a commuter though.
Wheel rim. Cheap rims are made of steel, with chrome plating. They are heavy and crudely made. The chrome pits and the steel rusts. Better rims are made of aluminium alloy which is lighter and doesn’t rust. There are different grades of alloy, varying in strength and (slightly) in lightness. Then you must consider the section of the rim. A deep section may hold the tyre bead more securely; some have an aerodynamic section (seldom relevant, but looks good!); some are wider, and can take a fatter tyre (better for grip, comfort, and tricks); some have a chambered construction for greater rigidity.
Spokes. A standard wheel has 36 spokes. Specialised wheels for heavy duty use have more, so are stronger. A cheap wheel is assembled entirely by machine, and the spoke tension is very approximate. A more expensive wheel may be either hand tensioned (better) or completely hand built (best of all). A hand tensioned or hand built wheel will take more hard use before it starts to go out of shape.
Hub. One end of each spoke goes into a flange in the hub. the diameter of the flanges can vary. A bigger flange makes a stronger wheel. The width of the hub can vary, meaning that the frame has to be the right width.
The hub is the point of contact with the cranks which lead to the pedals. On very cheap unicycles, the cranks are attached using cotter pins. This is most unsatisfactory as they work loose, and the pins can shear. Also, there is only a limited selection of cottered cranks available. Better hubs have square tapers on the axles, so that a ‘cotterless crank’ can slip over the end and be held in place with a nut (or bolt). The best hubs of all are splined (same cross section as a cog) and these are much more secure and durable if the unicycle is going to receive hard use, including jumps and drops.
The cranks can vary in quality and weight, with a choice of steel or aluminium alloy. Certain makes are prone to bending or snapping with hard use. Some makes are available in a bigger range of lengths. Changing the length of the cranks is one way of ‘tuning’ a unicycle to optimise it for high speed, or steep hill climbing, etc.
The pedals. many unicycles come from the shop with cheap basic pedals because it is assumed that the owner will fit upgraded pedals to suit. Pedals can vary in price from about 4 Pounds to over 100 Pounds. It depends on the design, the material, the weight, the quality of the bearings, and whether the pins 9for grip) are replaceable or not.
Back to the hub: it has bearings attached. These can be sealed or not sealed. Sealed are better.
The bearings can be held in place with ‘lollipop’ bearing holders, or C shaped clamps. The clamps are better. Lollipops can be hard to remove, and the can damage the bottoms of the forks.
The clamps may be made of stamped and pressed steel, or machined from solid metal, or even (I guess) cast. Machined ones are more expensive, but more rugged.
Then there’s the forks. Different grades of metal, with different diameter tubing. Sometimes the tubes are simple cyclinders; on better ones, they are shaped to be strong in the right palces and the right directions. The forks join at the crown, which may be a simple Y shaped butted joint, or a more complex shape with a place to put your foot when doing tricks. Some frames use lugs; castings which have sockets into which the ends of the fork tubes are welded.
Then there’s the seat tube, which can be (approx.) 22mm or 25mm. 25 is thicker, stronger, but heavier.
This influences the choice and availability of seat clamp. Cheaper unicycles have very crude quick release seat clamps (used to sdjust the height of the seat), whereas better unis have quite sophisticated clamps, sometimes with 2 Allen bolts.
Then there’s the seat tube, with a variety of qualities and weights.
And the saddle. Is your posterior worth only a cheap saddle?
At UK prices, 50 Pounds buys a cheap basic unicycle which a keen rider would soon find limiting; 125 - 150 Pounds buys one which will be light, capable and durable; 200 Pounds plus buys one that’s a bit special.
Apart from my first one, I’ve always bought mid range ones, then upgraded the pedals and cranks. I have 3 Nimbuses (‘Nimbi’?) one each of 20, 24 and 28 inch, and I’ve found them all satisfactory. The Nimbus 2 frame is more stylish, better for tricks, and worth a little bit more money, although some people find that the fork crown can catch the inside of the knee on the larger sizes.
I’ve stuck my neck out here on technical detail, so no doubt someone cleverer than I am will find much fault in the above, but at least it should be enough to explain the big dofferences in prices you’ve noticed.