First of all, No Intelligence Costs $100, not $50. Thats what Ginny asks for at the Start of part 2, I know because I just checked.
The Gym costs $50, not the Library. But why would I go to the Gym? The Big bad is there and he will simply beat the MC into another Reboot. Best to avoid that place.
Next: Using the computer in my room to work with my Int Stat I earned $19 after taxes. This appears to be the only way to earn money at the start (Thats after the very first scene but having done absolutely nothing else).
So As I said: It will take over 5 days to earn enough money to boost my Intelligence by +1.