Sometimes I think it makes sense for folks to do the planning themselves and sometimes it makes sense to bring in some outside help - it really depends on how quickly you need the work done and what people you have in-house to get the work done. I will say I think that you will understand your customer base and the value you deliver to those customers better than anyone on the outside does. A marketing firm can help you craft better messages around your value propositions and come up with a tactical plan for running lead generation campaigns. How much you will pay really depends on the size of the plan. The bigger your company is the more complex the lead generation plan will be (put it this way - I've seen plans done for as little as 5K and as much as 100K). If you are smaller you might want to work with an independent marketing consultant who will not only charge you less but can probably help you with the execution of the plan on an ongoing basis.