The major difference between Pay Per Click and Search Engine Optimization is that PPC is a paid ranking and and you have to invest in every click you want to get, while SEO is a way of getting good ranking, and with it more traffic in a natural, organic way. You may be wrong if you consider that SEO will be free of charge for you, as you can choose to outsource seo and hire a professional seo firm to perform all the work for you.  SEO price may differ depending of the company and the services provided.

The main benefit of PPC advertising is that it is instant, you can create and start a campaign in one day, whilst SEO is a time consuming process, which takes months of work and usually involves implementing some changes on your website to make it search engine friendly. You need to take some time before you see any results from SEO, but if you do everything right, you will be happy to see the improvement of your organic search engine position.

In contrary, PPC ads will be a permanent cost for for your online business over time, but will attract very fast results, which can be very useful for short term promotions, as there is much lower initial cost involved.

When choosing one or another online promotion tool, you would certainly give your preference to the one, which can later show you some tangible results. Here you can face some problems estimating the initial search engine optimization value against the initial cost of implementing it.
What can not be measured, can not be evaluated. PPC campaign can be assessed more easily as you will be dealing with your investments and profit in a relatively short term.

PPC advertising providers such as Google AdWords, Yahoo, etc. provide information on each and every ad performance.  No need to set up any extra tracking, as leads or sales can be tracked by adding a code, meaning that the cost per sale or lead can be tracked from the very beginning.
Quite the opposite, measuring the results of seo performance is a much harder task and requires an installation of website analytics package or log files.

Usually online marketing professionals suggest using both SEO and PPC, as one complements another and generate better results in a long term perspective. For example, once a website is ranked well within the search engine PPC can still be used for short term promotions, new keyword and key phrases testing and quick traffic generation.