ubersuggest, like the previous guy said...
Also, if you don't mind MOZ-based metrics, Keysearch is an option at $170 per year. It's what I'm using at the moment and it's decent. Of course that it can't compare to Ahrefs / Semrush but it gets the job done if your aspirations are not high
... But beware that Keysearch only supports USA, if you need to do research in foreign markets KS won't work...
KeywordTitan is another tool (I believe it's $200 per year or so) that I used in the past and which was good, but I'm not sure how updated it still is. Also, I think you need credits for some tasks and those credits can get expensive if you have a lot of SEO work to do each day...
Jaaxy is another one that I (briefly) used in the past (like 5-6 years ago), but I'm not seeing the price on the homepage anymore, so I can't say whether they're cheap or expensive. I think you'll have to sign up (free signup is allowed from what I'm seeing) and see the prices inside. But if I remember correctly their highest plan was $50 a month or so... But I might be wrong, it's long since I've used Jaaxy...
And finally, Mangools (ex-KeywordFinder I believe it was called) is yet another one that I've seen being recommended often. Personally, I've not used Mangools so I can't speak too much about it besides the fact that it's obviously cheaper than Ahrefs / Semrush... oh, and they also seem to miss lots of functionalities that the 2 biggest tools offer, but I thought I'd mention it anyway...
