^ This. Also, for synths specifically, I've been super satisfied using Hifishark lately. It's way more technical than the generic eBay trackers and pulls data from everywhere, including Reverb and private forums. You dont need an account to browse their Sold/Expired section which is exactly what you need to avoid those Buy It Now markups. I've noticed it handles the data noise much better than CheckAFlip because it filters for gear-specific categories. It works well since it lets you see global versus local pricing too. Since your budget is $400, that JU-06A is definitely within reach if you track the averages there... I've seen them move for $360 recently in great shape. No complaints about the UI either, its pretty straightforward for a data-heavy tool. If you stick to the sold tab there, you'll get a much more accurate technical breakdown of what the gear is actually worth right now.

TL;DR: Honestly, just use the eBay Sold filter manually. Most free trackers are laggy or flat-out wrong now. Be careful with third-party sites because they often mix for parts gear into the price averages. It makes a JU-06A look way cheaper than it really is. I would suggest double-checking every low price to make sure it isnt just a repair project before you trust the graph...

Having tracked gear prices for over a decade, I can tell you that CheckAFlip has become increasingly inconsistent due to eBay changing their API permissions. Most free trackers fail because they dont filter out junk data properly. If you want a reliable chart without an account, PriceBefore is technically your best bet but you must use negative keywords. A key tip for the JU-06A: always include -parts -repair -broken in your search string. Most trackers just pull raw title data, and those broken units will make it look like the synth is hitting your 400 dollar target more often than it actually is. Also, be wary of Japanese sellers with lower base prices. Most automated trackers dont factor in high shipping or import taxes, which can easily add 60 dollars to the total cost. Keeping the search localized is the only way to get a realistic graph.