It sounds to me like you may have already experienced the massive expense
by having a significant amount of work done that doesn't address the EVAP
problem. It doesn't do any good to take your car somewhere where they
cannot accurately diagnose the problem your car has. I personally don't
think the dealer is the only place that can diagnose an EVAP problem, but
it can be tricky to diagnose and does require special equipment.
Be sure to let the dealer know about the lamp going off when the tank is
empty, as well. Relax, authorize the diagnostic charge, and let them
check your car. If they're honest, they'll give you an estimate for the
work and let you know the consequences if you don't have the work done.
It's even possible that the repairs may not be that expensive. If you
want to shop around, find out specifically what needs to be done, and call
a few places. If they don't know what you're talking about, better to stay
at the dealer.