There is no answer to “best/cheapest”. Besides the fact that “best” and “cheapest” are rarely found together. We don’t even know what “best” means in your situation. And without a currency symbol and some digits, we don’t know what you mean by “cheapest” either. If “cheapest” means $50-100 then you probably have unrealistic expectations.
Is there some particular reason you are looking for a pin mic? There are very few choices (I know of only one, Rode). It is not an inexpensive solution.
Mounting a “gun mic” on the camera is a very poor method of capturing sound. That location is almost NEVER the right place for the microphone for many reasons (all of which have been discussed here many times before).
You are on the right track using an external sound recorder because the audio recording capabilities of a DSLR camera are generally too poor for practical use.
Yes, wireless is not cheap. The minimum-cost halfway decent wireless kit (lav mic, transmitter and receiver) is $500-600.
I have had poor experience recording with iPhone/iPod, and I have never seen anyone coming through this (or any other) forum reporting good results with an iPhone/iPad solution. Recording on iPhone isn’t really any better than just recording on the DSLR.