COBRA costs exactly as much as the total insurance cost when you were working. Almost all health insurance plans are employer subsidized, sometimes pretty heavily. So your 150 dollar a month plan could end up costing you 400 bucks a month to keep when you hit COBRA.

Edit: Perhaps I wasn't adequately clear. My point is that you are responsible for the ENTIRE insurance bill when you go on COBRA, your part and your employer's contribution. Sometimes that contribution is quite large and it results in COBRA bills that aren't affordable, especially when compared to individual insurance options.