Here is what I can generically say, rather than commenting directly on
your case in particular:

ISTALOL contains the same medication as timolol.  However, it is
formulated in a unique way (as TIMOPTIC-XE is) to allow once a day
dosing.  That is, essentially, the only real difference between
timolol and ISTALOL.

IOW, ISTALOL qd is promoted and marketed as providing *the same IOP
lowering* as timolol bid provides.  But in fact, timolol bid may
provide a more steady lowering and blunting of the IOP curve than
ISTALOL.

So, in general, timolol bid and ISTALOL qd are interchangable.  The
reason doctors prescribe ISTALOL (or TIMOPTIC-XE) over standard
timolol generally is because a once a day dosage promotes compliance.
Most (but not all) of the time, there is no reason why a person *must
be* on ISTALOL versus regular timolol.

If cost is a major concern, then I would askl your eye doctor two
questions:

1.  Why did he switch you from timolol to ISTALOL?

and, depending on the answer here,

2.  Tell him cost is a "big deal" and ask him if there is any way he
could switch you back to timolol.  If not, have him explain why.

FYI, one cannot say without knowing your optic nerve status over time
whether your glaucoma is/has been "under control" if the IOP is
"consistently about 18 in both eyes."  And there is no a priori way of
knowing whether in the future your glaucoma will not progress at an
unacceptable rate if your IOPs are consistently about 18.

It is important to understand that IOP is merely a risk factor for
glaucoma, as is thinner CCTs, age, family history of glaucoma, race,
etc.  However, it is an extremely imporant risk factor, because it is
the only one that eye doctors can (attempt to) control.

My eye doc has recommended that I swith from Timolol to Istalol for my
glaucoma.
Problem is that in my drug plan Timolol costs me $3 foe 90 day supply,
and Istalol is $76!!! It's not in their "preferred list".
Is it worth it? My pressure seems to be under control, consistently
about 18 in both eyes.