Once I have it working like I want it to, I program an ATtiny with the sketch. Then it can do the dirty work, and my Arduino is freed for other projects.
(ATtiny85 has been fine so far for all my needs)
The tinies are cheap, rugged and far smaller than a "full Arduino", and I simply stick them in a socket on a piece of vero-board.
And right you are, Peter_I. But the attiny85 and the like are a weebit challenged with I/O and SRAM and Flash. Many Arduino projects will scale-down to a tiny, but the programmer needs to keep the "tiny" in mind through the entire design and coding session. My last shipment qty25 of ATtiny85-PU cost me $0.89 each + shipping. (just checked and they are $0.85 U.S. from Newark.)
But, when consigned to a perfboard design, the per-unit cost of the tiny vs the full mega328 is a small difference: $2.08 - 0.85 or $1.23 or 145% more... but, you are almost certainly assured that the perfboard will work identically (sans USB) to the UNO you used for development. It really comes down to the number of non-reclaimable projects you build. I built many, many tiny85 projects before I finally decided to just standardize on the Atmega328P-PU microcontroller. Therefore, I buy more of the same chip and get a less expensive price. And, more of my code is reusable! And, it is very rare that I include an IC socket, so that cost-savings defers a bit more of the 328 expense. If the project is going to be permanent, the need for a socket is one of "insurance" and in some cases such as vibration, the socket is really a "downer."
My FLAT-duino design:http://www.instructables.com/id/Build-A-FLAT-duino/
In the case of the SD-Card logger, the SD card has a 3.3V regulator that powers the ATmega328. So a significant economy is achieved by not having to worry about the 5V to 3.3V requirements of signaling.
I still use the "tiny" when I need something physically small... often "dead bug" design