What is the typical form for a finished Arduino project? I just started with Arduino. I am an experienced software developer so the programming side was already mostly known to me, and I am surprised at just how easy Arduino have made the hardware side of things, and how easy to use the various libraries are.
So for my first non tutorial project, I want to make air quality sensor web servers to be placed in several rooms throughout my house. I will then have a program running on my PC which collects readings from each sensor and stores then somewhere, to be analyzed later.

All I have right now is an Arduino UNO R3 starter kit, and I am having a WiFi shield delivered today. I feel confident that I can have a working version using the kit that I have shortly after the shield gets delivered.

But my question is: what is the best (some combination of easiest and cheapest) way to crank out half a dozen of these? 

Presumably, most experienced makers don't include a breadboard in their finished products? I suppose I should buy a soldering iron.

Should I buy half a dozen UNO R3s and half a dozen WiFi shields (along with air quality sensors)? Or is there a cheaper option that is intended for semi-productionizing an Arduino project?

PS: I feel like the tags I have included barely intersect with the purpose of this question, so if you know of any more appropriate ones, feel free to suggest/edit.
I'm in the same boat as you (SW engineer by trade who likes to tinker with electronics, and is a "maker" at heart.)

I think the answer to your question is "it depends." For a one-off, a breadboard might be fine.

For half-a-dozen, you might want to get project cases and perf-boards, and solder up your design. (For the Mega, there is a very nice prototyping "shield" that brings all the pins from the Mega up on headers and has quite a bit of working room for soldering up circuits.) If you want to make more than a handful, though, you'd probably want to create a custom PC board. You can either create a design in a CAD program and send it off to be small-batch manufactured, or you can buy the equipment to do it yourself. If you make hundreds of units, you should think about ditching the Arduinos and designing a completely custom board using the AVR-family microcontroller and only the components you need.

(I'm still at the breadboard/hand soldered perf-board stage myself, and haven't created any custom PC boards yet, just read about it.)