Web development is a feedback process because clients don't know what they want and they ask you to build off of their feelings, not specifications. So constant communication with clients is required. If cost of this is high for you, then you may not find this service profitable.

The good thing about wordpress theming is that while not every client knows what they want, most clients will be happy with the same couple of things. For this, I recommend building a theme that could act as a starting point for all projects... I mean that has all the luxury features a client could want, so you just remove the parts you don't need later on.

Having your product set up ahead of time as I described above is one way to get around wasting time when you're working with clients.