No that would not be considered cloaking. In fact GoTo recommends the method you describe for tracking so that you know hits from partner sites are from your PPC campaign at GoTo, you can read about it [url=www.goto.com/d/about/advertisers/ustracking.jhtml]HERE[/url].
IMO the ROI of paying for clicks depends on many factors, there really is no right amount to spend, all you can do is project the traffic and extrapolate conversion ratios. These numbers will be different for everyone. Some do adopt a strategy of finding niche keywords and therefore are able to get decent targetted traffic at much lower bid prices.
You may have already seen it, in case you haven't the [url=www.goto.com/d/about/advertisers/usfaq.jhtml]FAQ[/url] at GoTo has additional information you may find useful.