For a cost/watt ratio clothing is hard to beat. Aero socks are cheap and a tight jersey doesn’t have to be super expensive plus you gotta wear something!
Position is obviously free, until you get into new narrow bars and short cranks but if you’re building a new bike it’s not extra per se just the build cost. It’s also not a quick win, being able to hold the bent arm posture for a while takes time and effort training but it’s always good to get stronger.
Things like wheels, helmets get good gains but they’re not good value. Deep wheels are a significant investment which brings the ratio right down. Helmets are expensive and very personal to fitting and gains so need plenty of testing to find the correct one.