I love the idea of a doula but my midwife isn't a fan
That seems kind of odd to me. Why doesn't your midwife want a doula there? My midwives recommend them, and gave me a list of people I could call if I wanted.

I haven't ever had one myself, but from what I gather, the midwife is there to watch primarily over your physical health and safety. She will offer emotional support and encouragement too, of course, but that's not her first job. The doula is there to give you encouragement, keep you in a good mind set, and offer suggestions for making you more comfortable. So their roles overlap a little, but should be complementary.

As for what they charge, again, I think around here you can get quite a wide range, depending on the level of experience and the type of service offered. Some will do a number of prenatal visits and post partum visits, and that will increase the cost, of course. Some offer special services like photography, or mementos of the birth. You could consider having a student doula at your birth, if cost is a concern. They have to attend a certain number of births before they are certified, and often do that a little or no cost.