Picking an AC-DC Converter to Minimize Cost
The cheapest option is to copy exactly what is used in the cheapest 2.1A cell phone charger and buy the same garbage parts from the same garbage sources and have them put together in the same illegal sweatshops. They're almost always flyback converter designs, because it's cheapest at low power levels. I've seen such things for &lt; $1 US shipped from China. 

For bonus points, save money by ignoring UL/CSA/IEC rules on clearances, ignore flammability concerns, avoid using X and Y mains rated safety capacitors, use insulation materials and PCB material from the lowest bidder (preferably returns), don't bother with any EMI considerations, and, since you'll fail anyway, don't bother with FCC and safety agency approvals. Hint: You can buy rolls of "CE" (Can't Enforce) stickers quite inexpensively. 

If you actually have needs beyond "cheapest" you could look at reference designs from Power Integrations, some of which are usable by a neophyte.