The real secret is finding the web developer (company?) that knows enough to ask you the right questions, to find out what your business needs are, and to come up with a solution that best matches your business needs, budget, and schedule -- remembering that all 3 of those are interconnected, and when you change one of them, at least one of the others has to grow/shrink in response.
Depending on your technical expertise, it may be premature to start with the assumption that Magento is the way to go -- it may, in fact, be a good choice, but it might be a bit early to jump to such a conclusion. A good developer may know of a better solution that better matches your specific business needs.
Long story short: the real trick is finding the right developer. Unfortunately, the better s/he is, likely the more s/he will cost. However, that increase may only apply in the short term: pinching too many pennies now may result in spending more total dollars down the road as you scramble to fix an initial mess.
Okay, enough rambling by me. In closing: