Plan for the low five figures rather than a few thousand, and know that the licensing piece varies enormously by state. Some states license home inspectors with required training hours, an exam, and proof of insurance. Some have no license at all and the only gate is what your professional association and your insurer require. Confirm your own state's rules with the agency that regulates it before you spend anything on training, because a course that satisfies one state may not satisfy yours.
The usual pieces. Training and exam prep runs somewhere in the several hundred to a few thousand range depending on whether it's online or classroom, plus the exam fee. Association membership is typically a few hundred a year. Insurance is two policies, general liability and errors and omissions, and E and O is the one that stings, often in the low thousands annually for a new inspector with no history. Report software is a subscription, usually somewhere around $50 to $100 a month, sometimes with a per-report option instead. Tools are moderate if you already own ladders and meters, more if you don't, and a thermal camera is optional at the start.
The cost people underestimate is the ramp. Inspection work comes from agent relationships and those take time to build, so your first year is often part-time income against full-year insurance and software bills. Budget the runway alongside the startup number, and think early about whether you'll add maintenance inspections for owners, because that work doesn't depend on someone being mid-purchase and it's easier to sell to people who already know you.