Pick a daily cost you’re comfortable with. If you’re contracting at say $500/day, how much are you willing to spend on having a responsive machine? $10? $20?

Pick a daily cost you’re comfortable with. If you’re contracting at say $500/day, how much are you willing to spend on having a responsive machine? $10? $20? Multiply it out: 220 work days a year * $10/day is $2200 a year towards your laptop. Upgrade accordingly.