Sadly don't have right now access to my latest exploration payouts so I'll refer to my faulty memory. IN general, fully mapping planets is very profitable. When I just FSS the system and see there;s nothing of interest I get few thousand credits (~3k per twin suns, no planets) up to few hundred thousands (170-200k per scanning just gas giants). If you add metal content and/or high metal content then payouts can pass a million credits. I saw 800k-1.2M for such scans.
Ofc Ammonia, WW and ELW are the best paying ones. Single FSS of an ELW is ~600k. Mapping it with probes raises the bar even higher. My recent highest payout was over 12 M per single system. It had Ammonia, 3x WW and 2x ELW, rest of the rabble I skipped.
One small system, 2x WW and single ELW + handful of rocky bodies gave me the highest payout so far - over 13M. I discovered that system over a year ago, after exploration mechanic change I went there to map it. Now this system is both discovered and fully mapped by me.
Overall fully mapping system is time consuming. If you're after credits focus on high value spots. Learn the wave scanner to easily spot metal content, ELW, WW, Ammonia and gas giants. FSS those, probe Ammonia, WW and ELW. I am doing this on my current exploration journey, made more 10k, less than 15k LYs and so far earned 128M. Solid payout, not theoretical since I sold that data. Could be more but I skipped on some planets, sometimes you don't want to travel 380k Ls for a single WW to map.