Trailer is about double the cost of a dolly. It also depends on what you are using to tow. If the tow vehicle isn't heavy enough they won't even rent you the trailer.
As far as the width is concerned, there is no concern. My 3000GT's and Stealth's are wider than the Grand Prix and I never had any real issues getting them on a dolly. The only time I had any issue was putting my own Stealth on a dolly. It's lowered on coilovers and I had 9" wide wheels on it. Once I finally got it up on the dolly itself I had to let air out of the front tires to get it to sit in the troughs. Once strapped down though I had no issues towing it the 70 miles home.