Start with the market.
Market size and local competition shape the amount and cost of available traffic. A crowded metropolitan market and a smaller service area should not automatically receive the same budget.
Factor in your practice area.
Different legal needs carry different search demand, competition, and case values. Be specific about the work your firm wants rather than funding a vague campaign for every legal service.
Connect cost per click to cost per case.
A low click price is not a win if those visitors never become clients. Cost per case means total advertising cost divided by signed cases attributable to that advertising. Look at qualification and intake conversion alongside the price of traffic.
Fund a test you can learn from.
A sensible test needs enough relevant traffic to reveal patterns while staying within a budget your firm can sustain. Agree on what success looks like, track inquiries through intake, and review the evidence before scaling. No budget can guarantee a particular outcome.