if you are going to keep this car for long term, change it. if not, dont change it and sell it. i dont think just the timing belt would cost 700.

it is wise to do multiple work in your car at the same time to save on labor. usually, timing belt, another belt, water pump are changed at the same time to avoid paying for multiple hours of labor. again, it also depends on the model of the car and where exactly those different parts are located. anyway, 700 is too much just for timing belt, seek second opinion.

maybe post your location and see if any sajhabasi knows good mechanic in your area.