Board at my barn is $300/month for pasture board, with grain, hay, blanketing, fly masks, and minor first aid (if needed) included. Stall board is $400 for all of the above, plus stall cleaning 2x daily and turnout if desired. All boarders get to use the trails, schooling arena, jumping arena, etc.
I agree that it depends on what boarding there involves - someone with a fancy jumping arena or indoor arena could charge more because of the draw to it.
I would say $300 is probably a fair price, I'd maybe even go for $400. But, again, it all depends on what the other options are. If she's the only boarding barn in the area, she has the market cornered. If not, her competition might have more benefits, thus causing her to lower her prices to combat it.