the only reputable known signal boosting device is the Sprint AirRave
basically its for home use, costs about $50, connects to your wifi, and costs 15 bucks a month, but will pull you out of roaming and into full signal strength.
here is a useful hands on review:
http://www.engadgetmobile.com/2008/0...airave-review/
as for spending $500 on a signal booster...don't. You could probably erect your own cell phone tower for less.
[/nonesense over]