Of course that's a whole discussion but perhaps in the $300 range. Sounds like that's what Dr. Van Etten herself was using when she first found one.

She actually made some specific recommendations on the microscopy sub-reddit but they're a bit pricier ($425+). (scroll to last comment by Jay211TF):

https://www.reddit.com/r/microscopy/comments/1dhagyr/entry_l...