It's more like your paying them for what they know... hopefully based on an educated look at your company & site.
There is a story/tale I heard once and it's very fitting:
THere is this company and hey had to call in this retired person from the company to fix this issue after going though the people who replaced this person. The guy walks in and flips a couple switches and gets out a tool out of his bag and comes back in a few minutes saying "it's fixed and here is my bill for 30k" The owner says "30k What they heck is this!!!" The retired gentelmen says " it's 5 dollars for the part and $29995 for knowing it had to be replaced"
Moral of the story is if the company is truely worth it you'll see the results and not all things can be done in house.
I might also suggest going to whomever developed the site and ask what they think as well... they may cut you some sort of break being they developed the site.