There is no easy answer when it comes to deciding whether or not to fix an old mountain bike. On one hand, if the bike is sentimental or has significant meaning, then it might be worth fixing regardless of cost. On the other hand, if the bike is simply outdated and no longer fits your riding style, then it might not be worth fixing at all. Ultimately, the decision comes down to a personal assessment of both the financial and emotional value of the bike.
Assuming that you are primarily concerned with the financial aspect of this decision, there are a few things to consider. First, you will need to evaluate the cost of parts and labor needed to fix the bike. This can vary significantly depending on what needs to be fixed and how complex the repairs are. In some cases, it might simply be more cost effective to buy a new bike rather than repairing an old one. However, in other cases where only minor repairs are needed, it could make more sense financially to fix up the old bike.
Another important consideration is how much use you expect to get out of the repaired bike. If you plan on using it regularly and for many years into the future, then it might be worth investing in repairs now so that you can continue enjoying your ride for years to come. However, if you think you will only use the repaired bike occasionally or for short rides close to home, then it might not be worth spending as much money on repairs.