Mean Well has some really nicely priced 5W PCB mountable power supplies. You can find them for ~$13 in single quantities. I'm not sure you're going to find too much less than 5W.
Otherwise you could use a transformer, bridge rectifier (Two dirt cheap 1N400X rectifiers will work if your transformer has a center tap, otherwise go with 4.), two filter caps and a voltage regulator like an LM7805.
At 100mA, you probably wouldn't even need a heatsink for the LM7805. Switched Mode would require a slightly more expensive setup (You'd need an inductor, a switching regulator like an LM2575, and a flyback diode, totalling ~$4 extra). If your input is constrained to 120V, then the LM7805 may be the way to go. At 500mW, you wouldn't even need too big of a filter cap. You could get a 6.3VAC transformer, which would further reduce the size of it (And slightly increase the power wasted by the 7805, but at 100mA, it's not much.)