For the hobbyist, one great source of low cost electronic gizmos is ebay. There is a whole range of sellers, often based in Hong Kong, offering small mounted PCB for less than cost of the components bought in small volumes.
Examples of hits when searching for Arduino range from SD-card slots, wireless transmitter/receiver pairs, ultrasonic distance sensors, relay modules, real-time clocks, all the way to complete robotic arms or kits with Arduinos and a rich selection of add-ons.
More trivial product such as cables and breadboards can also be found to very low prices.
A personal favorite of mine is the 3.3/5V breadboard power supplies that can be found from various sellers. Being able to get a stable voltage, either from USB or a small power supply, without having to carry an actual lab power supply is a great freedom.
Given that all this is available for practically no money at all, what is the cost? One is delivery time. Another is production conditions. A third is that the material usually is shipped without any manuals or reference documentation. A little digging on the Internet based on components located on the products is not uncommon. Still, compared to producing custom PCBs in small volume, the time spent and price is hard to beat.
Notice that Digital Fanatics in no way endorses any products or individual sellers. All product links are just examples. Any purchase is made at your own risk.