Audio falls as an afterthought so often, and after a $1000+ investment in a camera, lenses, batteries, etc., people want to know how little they can spend on audio gear. Folks, good audio is not cheap and cheap audio can cost you a lot.
Also, wireless audio is not a first solution. Use a boom mic, or a hard-wired lav, whenever possible. But there are some times that you need the flexibility and mobility that wireless offers. Once you've determined that you do need to get a system, here are a few things to know about cheap wireless systems, and wireless systems (aka radio mics) in general:
2) The stock lavaliere mic that comes with the system is probably sub-par, so plan on an extra $150 - 400 for a lav upgrade. (There are a few lav shootouts and reviews around here, so be sure to check those out.)
3) It does not matter how much you spend on a wireless system: every radio mic will fail at some point. It's just that some systems are stronger and more stable than others, and strength and stability come with financial investment.
4) Buying a cheap system is going to result in audio nightmares for you both during production and in post.
5) No, you can't just buy an extra transmitter and use both transmitters with one receiver. You must have a second receiver with the second transmitter, and run that second system on a different frequency. Yes, there are systems that come with dual transmitters and one receiver unit, but that unit has two receivers built in along with two separate audio outputs.
The Sennheiser Evolution G3 ENG system runs around $600 per channel as of this posting. This is by far the leader in price-to-performance for budget wireless. The predecessor, the G2, was also a very reliable system. Beware buying on your favorite auction site. There were floods of Chinese counterfeit G2 units, so know your seller.
Also of note between the G2 and G3: the G2 ENG systems are not diversity units, meaning there is only one antenna to receive the signal; the G3 systems added diversity (a second antenna, reducing the likelihood for signal dropouts) by utilizing the audio output cable from the receiver as the second antenna.
The Audio-Technica 1800-series wireless systems are around $500 per channel as of this post. They replaced the older U100-series systems, which were very good in the sub-$1000 category. There is a dual-frequency system available in the 1800 series that has two transmitters and a dual-channel receiver. Though the U100 systems were splendid in their class (I had two of them), I have not seen or heard many positive reviews of the 1800 series (I have no direct personal experience with the 1800 systems). These systems offer diversity reception.
The Sony UWP-V1 systems are the third contenders. At about $500/channel as of this post ($450-525), they offer diversity and a surprisingly long range. I demoed one of these systems and was not impressed, however. There was a noise floor from the unit, and the noise breathed with the compander when the transmitter was not muted.
Of the three systems mentioned above, the G2/G3 consistently proves a champion and is the not-quite-universal recommendation.
As with so many things in gear, you get what you pay for. And if you cannot purchase a reliable system now, consider renting on a per-project basis. Equipment rental costs can be passed on to paying clients. Buying a cheap wireless now "until I can afford something better" will actually do you more harm than good, and that's money wasted. There are rental companies that will ship anywhere, and a system can be rented for a song. Weekly rates make it even better.
So what's really wrong with the cheap wireless systems? Unreliable transmitters and receivers, cheap construction, poor companders with deplorable frequency response and hideous noise levels. Often, the uber-cheap systems run on VHF frequencies and are non-diversity, which means terrible reception and more vulnerability to interference. UHF diversity is your best bet. And if it says 2.4GHz on it, don't even bother (I'm lookin' at you, Line 6 Relay).
When you're ready to get serious about wireless audio, consider something from Lectrosonics or Zaxcom. Here begins the $1000+/channel range of wireless, but keep in mind that these offer industry-standard levels of performance and can be found on professional, high-budget sets and locations around the world. These also rent for very reasonable rates.
As a side-note: if you decide to purchase a used system, please do your homework. As of June 12, 2010, the FCC prohibits use of radio mics in the 700MHz band. There are wireless systems in that band that still pop up on the used market, so be sure to check when you buy used and make sure it's still legal.
One last thought: it does not matter what project you're working on, whether it's student projects or indie films/shorts or wedding/event videography or ENG/EFP or full-blown feature productions. These rules do not change.
Best of luck!