From experience, for 12.04 and up almost all webcams work out of the box. I have tested the following and they work perfect:
Logitech C200, QuickCam Pro 3000, Pro C920, C270, C310, C615 and a couple others I do not have with me nearby.
Genius Slim 320, Eye 320 and 2 more that I do not remember ^^.
HP HD 4110 and one more but is not HD.
Apart from Logitech, Genius and HP I have had others that have weird names so I do not remember them.
Even though the list here: https://wiki.ubuntu.com/HardwareSupportComponentsMultimediaWebCameras mentions a group of webcams, there are many more that are not there and actually work out of the box.
There are even lists in the web like the following which list an amount of webcams that work (Although this sites are not updated regularly):
but you have a very good bet that the webcam you buy will work.
One thing I do have to add is that, for HD webcams that have buttons, for example, to take a picture or open a webcam app, this buttons do not work for the moment. At least on 2 Logitech I tried and an HP. Apart from that they do offer resolutions at 1080 and are perfect.
With regards to the price, at least several that appear in this list I have already tested and work out of the box: http://www.amazon.com/Webcams-Computer-Add-Ons-Computers/b?ie=UTF8&node=172511
And for 25$ or less several here I have already tested and also work: http://www.amazon.com/s/ref=sr_nr_p_36_0?rh=n%3A172282%2Cn%3A!493964%2Cn%3A541966%2Cn%3A172511%2Cp_36%3A1253503011&bbn=172511&ie=UTF8&qid=1347329616&rnid=386442011
Do not pay attention to the mention of "Works with Microsoft Windows". Most hardware works on Linux without the need to install additional drivers.
What I do recommend is after buying the webcam you need to install
cheese to test the webcam out. I like cheese because you can change resolutions for video and pictures and add effects to the webcam and see how it performs.