I hope this is more informative.
Paul, there are different variables that has to be consider to bring your site on top. These variables are CTR [Click/Impression], Ad Copy Quality Score, Daily Budget - CPC, Landing Page and few other. [This is only in case of google adwords and not in yahoo overture]
But if you see there is a chain reaction to everything. to begin to this chain reaction you need to have a click - to get a click on your ad copy, you need good copy - to get visibility to the good copy, you need to be positioned high - to be positioned high you need to increase the cpc / daily budget.
Ad copy quality score can be acheived if you have less competition on a given keyword with a proper visibility on the first search made. for that ad position can be between 1.0 - 1.9 to 2.0 - 3.0
I hope these information is useful.
Regards
Ryan
it takes more than just paying more to get the top spot.