Is it permissible for me to perform Hajj on behalf of my parents, given the high cost of Hajj from Tunisia and the difficulty of affording it, while it is affordable from my current place of residence?

It is not permissible to perform Hajj on behalf of another unless one has already performed Hajj for oneself. This can be done on behalf of a deceased person or someone who is physically unable. The father is obligated to perform Hajj for himself if he is able.