Hi,
Amazon is known to change prices fast. That's why eg for Opera there exists plugins which watch prices on Amazon.
Moreover Amazon could do kind of A/B testing which means that "some" users even may see a different price. I don't know whether it happens or not - but if you want the perfect system you want to catch those cases.
Your goal is to be cheaper or as cheap as always. Thus to reach your goal it may not be enough to watch pages once - it may be required to run the same query 10 times eventually from different locations.
Also you didn't mention whether you care about the "second hand" price or the price for new products. Amazon offers both.
Just writing a script which gathers prciing from amazaon (via http://) is pretty easy.
Thus if all you need is getting all spans with class "price" and running an amazon query its less than 2 hours of work => maybe $80.
Now it depends on how much additional bookkeeping has to be done, eg you can't gather everything at the same time, you have to split the "list" to be done into pieces etc.