Generally political advertisements, campaign spending and especially donations are strongly limited. E.g. where I am a only private individuals can only donate and only up to 10k per year. The downside of that is that most funding parties receive comes from the state and it's based on their previous election performance (which is problematic due to very obvious reasons...).

Also if everyone can give/spend as much as they want legally it at least stays semi transparent so in theory voters can base their decisions on that. Illegal bribes, kickbacks etc. are a bit harder to track.