Well that's very good if you're happy. I'd say 300 is worth her time alright.. it's not cheap so you should get a nice plan for that, possibly even a few different options.
But even if you don't get several options at least you got a person you consider to be an expert to call to your house and offer her expertise.. so it's grand if you think so. As I say I'd expect an interior designer to charge from 200 to 500 for a kitchen design.. depending on how much work there's to be done.. eg.. some interior designers would source products, and provide samples of paint colours, or fabrics in which case they'd charge more than if just doing a drawing.
Maybe that woman will also be able to help you get a better price for some parts you need...