A bike is not a simple toy. Its a means of transportation, a complex piece of machinery that has evolved over a hundred years. Today there are many types of bikes to suit all users and all purposes.
A basic geared bike with no accessories consists of over a thousand pieces. As well as the frame and forks amongst the parts going into our bike are the stem, handlebars, wheels, tires, inner tubes, rim tapes, brake levers, shifters, grips, cables, headset, bottom bracket, chainset, cranks, pedals, sprockets, derailleurs (front and rear), chain, brake arms and springs, brake pads, saddle, seat post (and clamp) and more. Some of these parts are in turn made from many other smaller parts, and this is just for a basic bike.
Let's consider just one of these for a moment: the bicycle wheel. What an incredible invention. Light, strong and versatile. A wheel consists of a hub (made from many smaller parts, see the image), a number of spokes, nipples (these screw onto the ends of the spokes) and a rim. The hub needs to be correctly set up so that the axle rotates smoothly within the hub shell with no play and minimum resistance. Spokes need to be correctly tensioned so that the rim spins round in a straight line with no 'bumps' or 'hops'. Additionally a rear wheel needs to be 'dished' to compensate for the sprockets on the right hand side of the hub. If the wheel doesn't run true the brakes can't work effectively and the bike won't ride well. And this is just one wheel, one piece of the final jigsaw. A bike is a complicated piece of equipment. So for those of you that might need the point clarified; a bike must to be built well in order to run well. The parts need to be correctly assembled, bearings greased, components aligned, bolts correctly tightened, cables tensioned, braking and gearing systems correctly set up, etc. This is a skilled process.
Add shipping, advertising, profit(!) and all the other activities involved in running a business and you finally get to the cost of the end product.