Tax preparation fees at my firm are primarily based on the complexity of the work involved. As the complexity of your returns increases/decreases, so does your fee.
You should select a tax advisor that is not only competent to prepare your returns, but one that can communicate with you in a way that is suitable for you understanding what's reported on your returns. Selecting an advisor based on the lowest fees might mean that you end up costing yourself much more in the future. Correcting prior year returns is much more expensive than doing them right the first time. Also, penalties from taxing authorities can be substantial.