Amazon has many e-ink displays for under $100. I am going to order a Waveshare 4.2" display with a resolution of 400 x 300 pixels. I chose this product because it has an SPI interface and it says it has example software for the Raspberry Pi. I could have chosen a 7" display.
This display, like most e-ink displays are black and white (and red in the case of the module I ordered), without grayscale. I did see one display that had four levels of grayscale, but I wasn't as confident of the interface. This means that you'd have to avoid web pages that have fancy graphics.
I don't know if they have a driver that would allow this display to be used like a monitor. If not, you'd have to use modify a web browser to buffer the image, modify the image to be a true black/white image, then show that image through the SPI interface to the e-ink display. There is a simple web browser written in python that you could modify if necessary.
The other problem for using this as an interactive display is that it doesn't have a touch screen. I don't know how well a mouse can be displayed on it, even if there is a device driver that would allow it to be used as a monitor.
E-ink displays are great for uses where you have something specific to display that doesn't change often and you need something that has a low power requirement. They have a low refresh rate and they are more expensive that a more capable traditional display would be.