Honestly, you generally won't find a quality wireless headset that works with the Mitel phones for under $200, or at least not much under that, these phones are not like the Aastra 39i that has built-in Bluetooth or ESI phones that have a sub-$100 BT module you can get.
The Plantronics CS50, CS55, CS70, or CS520/530, Jabra 920, or AT&T TL761x series can all be found on Amazon for around $170-$200 with the lifter kit (required to answer the phone or make calls properly without touching the phone) or about $40 less without it. On eBay most of these can be come by for around $120-$150 from reputable dealers, but be wary of the ones from China or Hong Kong, they are not always what they appear to be.
That being said, eBay sometimes has some kind of good deal, right now there are some new GN Netcom Jabra 9120 headset's with lifters for $99, they are an older model but work well with the Mitel 53xx series phones.
http://www.ebay.com/itm/NEW-Jabra-GN-9120-LR-Wireless-Headset-with-GN-1000-Remote-Handset-Lifter-GN9120-/170973255452
