As you progress through the game you will earn money from capturing ghosts, killing creatures and saving the day. Save up enough money for your upgrades, I suggest upgrading your Proton Pack as soon as you can followed by the slam trap. You will earn plenty of money through the campaign so there is no need to worry about missing any of these upgrades.
PKE Meter Scan Improvement - $5,000
PKE Meter Recharge Booster - $8,000
Fast Trap - $5,000
Slam Dunk Trapping - $18,000
Total - $36,000
All Upgrades Total: $166,500