媽有層樓想加兒子名,以長命契方式,因不等於50/50 OR 20/80 份額,用這方式可否省厘印費?100蚊厘印OK OR 要幾多? THANKS?

You will end up wasting more money than you can imagine.

In order to form a joint tenancy in this case, your mother must transfer the property to a non-party first, as all interests of the joint tenancy must be accquired at the same time.

So in order to do what you want - you will have to pay stamp duty tax in 2 occasions.

In answering your questions:

1. No.

2. The fair market value of the property X2

A cheaper alternative is a will and a power of attorney.