It's great to see more support and encouragement for women in cycling / mtb'ing. Currently, out of all the possible "outdoor pursuits" (such as climbing etc), cycling has the lowest proportion of women.
Women only events are a great idea for a few reasons. Cycling is so male dominated that a lot of the time I'm the only girl. Even on the normal Saturday ride with my local road club I'm the only woman riding with 40 guys, so often cycling is "men only" by default. I would definately ride with a women's group or event - not all the time, but sometimes as it would be good to have other girls to share experiences with, and to compare my performance with others of the same gender.
The £5 less is probably a gesture to try and encourage more women to take part. As for whether that is fair, be aware that women pay a "price penalty" for being such a minority in cycling. There is very little choice in women's cycling kit, so often we have to pay for a high priced product as we have no option to choose a cheaper alternative. Plus, if you wanted to go into wider societal gender differentiation, thd average women's salary is lower than men's, so we have less money per se.
Personally I love cycling / mtb'ing, and happily keep doing it even when I'm the only girl. The guys I ride with are great, but I do wish there were more women in cycling so fully support any attempts to encourage greater participation.