What is an inexpensive secondary insurance for my 95 yr old mom?

This is a very state specific question.  There are 4-6 states,  primarily in the North East that use "community rating" so your mom can get a Medigap policy.   In most other states, it will be next to impossible to get her anything other than Medicare Advantage, which when an elderly person is seriously ill, will likely limit coverage.  They will not give the elderly person 100 days in SNF (skilled nursing facility, AKA nursing home), but more likely 2-3 weeks.

200.00 monthly for a Medicare supplemental insurance policy is inexpensive and I wouldn't change anything at this point.

You say only dementia, dementia can change things for her overnight. It progressively gets worse and that causes many issues that can't even be imagined.

Is Kaiser available in your area? If not I know of no others.

travels4suzi, due to your mom's age and dementia, it's highly doubtful that she'll be able to get inexpensive secondary insurance. Assuming your mom has Medicare and given that Original Medicare doesn't have an out-of-pocket limit, I suggest she enroll in a Medicare Advantage plan, most of which have an annual out-of-pocket limit of a few thousand dollars, include Part D for prescription drugs, and some have no monthly premiums while others have lower premiums than what your mom is currently paying. You can find and compare various Medicare Advantage plans at www.medicare.gov  The annual enrollment for these plans ends Dec. 7, about 5 weeks from now.

Also, if your mom is a veteran or was married to one, don't overlook the healthcare benefits and financial aid available to veterans and their spouses.

Good luck and best wishes.