The best cost-benefit IMO is a glucose blood meter. I have a keto-mojo, which was fairly cheap (around 50 bucks at the time), works well, is small and measures ketones.
This way you know for sure when you are in keto, when your blood glucose starts decreasing, if your body has insulin responses to certain foods, and other related facts.
For example, one thing that I learned about myself is that 1) my blood glucose REALLY spikes in the morning, much more than I see for other people, signaling that I should avoid breaking my fast in the morning, 2) that my body holds my blood glucose low well, but it had a tough time reducing it. So I really need to jumpstart any fasting regimen with a 2-3 day fast at least.