You technically don’t need a lawyer for a civil divorce, either, but I wouldn’t recommend going into the process without one.
It is my understanding that it is easier to get an annulment from some bishops than it is from others (although I would welcome statistics one way or the other on this subject), so the extrinsic costs are likely to vary.
I only know about Los Angeles, but it is extremely rare there for somebody to hire an outside canon lawyer— like one case a year. Most applicants are poor or middle class. And it looks to me like a majority of applicants are women. This is troubling. It means that when men divorce they are walking away from the church entirely. It’s the women who want to keep attending mass.