I think you also have to look at it against the quality of organisation and the facilities you get with it.
In terms of races, MM has some of the best showers of any event I've been to, and it's well laid out with camping around the course and a key central area. I'm not a fan of the course however, but understand the need to make it rideable for everyone, particularly in the middle of the night. The Gorrick 12:12 last year was also very well organised for a first event, and the G3 Revolver series also went well. Riders get a reasonable 'race pack' and there are some nice asides like beer at the end, or top-caps with the race logo.
On the stage race front, Trans-Wales isn't THAT expensive, but the organisation last year needed to be better, and I hope they remedy it for this year. The riding was fantastic though, even in the inclement weather. For organisation, I think the Cape Epic is the one to beat - utterly amazing given the amount of competitors and the varying ability levels, and the distances covered. It's also very good value.
Organised rides need to be looked at separately. Some of these can seem like great value, and audax's are a lot better value than most MTB ones. Equally, a short hop over to France really shows you value for your entry fee.
At the end of the day, one of the defining things about any event is the 'atmosphere' and that is a pretty hard one to buy in. Clear descriptions/marking, good routes or courses and effective organisation at the start/finish go a long way to making the event efficient and worthwhile for most competitors, and then a few memorable sections en route can leave riders delighted.