Well, TyphoidHippo and Jimmy69 have summed it up--we did indeed have more info than was available in this thread alone (and it's a good demo as to why multiple threads on the same topic are a bad idea!).
A few comments on CSP's other points.
First, I think my comments on the "professional" mic were aimed more at the use of that word itself. I certainly don't argue that cheaper mics aren't capable of very good quality--I just take issue with the over-use of the word "pro" by salesmen or in model names. In this case, the OP had been sold the expectation that his $70 purchase is of the same quality as a mic costing 20 or 30 times as much. His $70 mic may be capable of good results (and, more important, it may be capable of the sound he wants) but it's just plain a lie to christen it a "professional mic".
I'm not anti cheaper mics--as regulars on the forum know, although I have Neumann and AKG mics in my collection, my present "go to" mic is the sE2200a--more than $70 but a heck of a lot less than my more expensive stuff. Similarly, I do a lot of live sound in theatres and, some years back, stopped using expensive MKE2 capsules, choosing to use
THESE or similar. A bit of EQ and they don't sound all that different to a £200 capsule--and the ability to treat them almost as consumables means that you're not struggling with mics that sound different after a couple of weeks because of sweat getting into them.
However, one of the drawbacks of cheap Chinese mics is the consistency. Some sound very nice indeed--but the next one off the line may sound very different. It's luck of draw. They also tend to not last as long--and breakdowns (especially in live work) can be embarrassing! Also, although the AUDIO side can be adequate, I've yet to find one that had acceptable RF performance, particularly when you're using multiple channels. The last time I checked the RF from a cheapy (a Nady as I recall) the filtering was so poor that the transmitter splattered across several adjacent frequencies--both illegal and also awkward when you're trying to cram in 22 channels! However, if you'd like to PM me, I'd love to hear the details of your Aus$100 radio mic! I'm always on the lookout for the holy grail of "cheap and good"!
Finally, on the wiring of the XLR to 1/8th inch adaptor, the incorrect wiring you mention should result in total cancellation, not just low levels. Although I can see circumstances where it could be as you describe, when I did this experiment for some teenagers I was teaching some years back, the cancellation was total! However, even with a properly made adaptor, I've yet to find a mic input on a modern computer that wasn't horrendously noisy. I went through a phase of having to feed voice tracks from Aus to the BBC in London via Skype. My first experiment was a decent mic into a PC with an adaptor cable--and it sounded as the OP described. In the end, I went with a solution involving a proper audio interface and the results (when I heard them back on air) were pretty good--still bandwidth limited but no extraneous noise etc.
