Search engine marketing is not a commodity.
While there are vendors who will position search marketing as such, it is not a commodity.
Search engine marketing is a service. It is a living, breathing, adaptable, evolving service.
This is not a one hit wonder.
This is not a gun it and run it.
Selecting a search marketing partner based on price alone could be something of a gamble. You might get great results. You might not. Either way, you probably don’t have any assurance of continuing service. There’s likely little chance of having a partner, essentially on staff, that you can work with to plan future marketing strategies. To foresee and react to changes in the search engine algorithms. To understand which evolving niche engines could best serve your business. Your entire search marketing strategy will probably stay reactive, rather than proactive.
The lowest price tag might look good on the bottom line, but only the first time you pay it. Will it still be a discount when you’re on your third project in a year?