It's not the money for entrance... well that's a lot but not all.
The trouble is the 'rest' imo. I don't mean the beer and chips, you buy that whereever you are, but the transport and lodging.
Some cities you know someone, who is capable and willing to take you and your party in house, otherwise you have to get a hotel since staying up all night never tends to be as fun as it should be. That in mind it's reservation time before the hotels notice the Stones will be in town, cause otherwise they rip you of saying it's exhibition-time at the local exhibition center(s) (it's usually in their conditions that they can raise prices then).
Transport is a thing too. I like trains as cars are a pain at that time. But not every country has small-groups discounts (Germany does, Holland is expensive, Belgium is cheap if you're willing to go more than once, and cross-border travelling is always expensive.) Even if entrance is only €20 the rest might trick you.
I was never in the position to go to more than two or three concerts. I accepted that. It's too much bother anyway dealing with hotels and transport, trading off possibilities and prices.
That way I never got in the 'habit' of wanting to be everywhere. Still I'm a little worried. You never know wheter you'll still be having a job when tour comes... Still I'd rather not eat for a month than settling for Bob Dylan.