Problem is that bare Wi-Fi modules are $15 to $25 even in thousand unit quantities. Add to that the higher level network protocols pretty well consume an Arduino class MCU, so you have to upgrade to a more expensive one. So now you are at $22 to $32 for just the core components. By the time you flesh it out, manufacture it and tuck in a little profit you are pretty much right where the Spark.io is, $39 retail.
That said, if you are creative (and will compromise) and can wade through the details of the docs, you don’t need the MCU if you use a bare RN-171. It has the IO pins to do what you want. That puts you back down in the $25 range in quantity, but you are still paying $35 for just one from Sparkfun or $27 for a raw one from Digi-Key.
As others have pointed out, XBees or other 915MHz radio modules (Moteino from lowpowerlab.com, Jeenode from jeelabs.com) are much cheaper but you will need to build a gateway to get them on the internet. Two of these are still cheaper than one Wi-Fi device.
Spark.io is on track to build a cheap usable unit. If even marginally successful their products should get more integrated and cheaper over time. If you are building a one-off device, pay the $39 and be done with it. If you intend to sell them or just use lots, then you can either pick a cheaper radio or wait for prices to come down.
Good luck,
- Chip