Id probably give her a $100-200. Depending on what a doula goes for in your area.
That would be a nice tip for her. Cheap for you (compared to full price). Shes doing the same job as a full price doula and you don't have to worry about finding a gift and wondering if its enough.
I'm mobile.... sorry for the typos!
Amanda- wife to DH. Mama to W (2010), L (2011), H (2013) & expecting #4 Jan 2016