For UK readers
Hi Viz vest from Poundworld (cost £1)
Includes 2 reflective stripes that go all around
Comes in 3 sizes I think (small, medium, large)
I got this hi viz vest out of Poundworld. In some shops these used to be ridiculously expensive (some still are) but I guess the likes of the pound/dollar shops smashed their monopoly. It's only £1 but it seems decent quality. To have the 2 reflective strips all around is great. Its 100% polyester and can be washed at 40 degrees a maximum of 25 times. This one is a 'large' size. Plenty of room. Great for commuter cycling. In the UK if you are going to buy a hi-viz then this one should certainly be considered. I don't like 'full jackets' or suchlike. It is also 'meshlike' (albeit, quite close meshing) which should help with sweating.
update : Nov 2015
Apparently UK trading standards forced it's withdrawal in 2014 as their tests suggested the grey trim has almost no reflective property.Daily Mail article Nov 2015
I did a quick photo test using a torch comparing it to Boardman overshoes and a Decathlon arm strap. my 'test' was inconclusive (to me)