I'm new to screenwriting and don't know which software would be the best to write with on a MacBook Pro, I know there are a huge number of programs to use but, I'm somewhat stuck on which one works the best with the cheapest price?
They still do a free version but it only allows you to create 3 screenplays for free and you lose some of the more advanced features after a certain time (think it's around a month). For a start-out screenwriter though, all the features you need really are still on the free version.
There is a workaround for the limit of 3 screenplays too. If you download the Celtx app on your phone, it will allow you to create additonal screenplays on there over and above that limit. Once you've created it on your phone, you can log back in on your computer and carry on working on it from there with no issue.
I started out with an old IBM Manual Selectric I picked up at a yard sale for $20 then eventually graduated to ScriptThing (still have several of the old ScriptThing t-shirts) which eventually turned into Movie Magic Screenwriter... Now I use Script Studio and before it changed its name to that, I used Movie Outline (same company) and have been using it for over a decade now but I have EVERYTHING on my laptop just in case I have to work on someone else's file. I've used them ALL and they all work just fine.
I use Script Studio because I can incorporate my own structure into it easily and effectively. It's like Scrivener for Screenwriters and even Scrivener might be worth looking at for your purposes since it's pretty cheap and does have a screenwriting feature that saves files as Final Draft files. Huge learning curve but you can also incorporate your own structure into it really well.
Having said that...
You can still find older versions of Celtx online for download that work fine... Those older versions were always completely FREE. I guess it would depend on your particular Mac OS when it comes down to which version to download. Just perform a Google search for it.
My favorite FREE screenwriting program is actually Trelby... Unfortunately, there is no Mac version.
A few years ago, I had a scriptdoctering gig and the writer used DramaQueen to write it. It's not bad at all if you like FREE.
Fade In has been the best one so far for simplicity and I like only paying once as opposed to having monthly payments for Adobe programs, scriptwriting programs, Filmfest programs, IMDB listings, etc... etc...
And that's part of why I've been happy with Final Draft. You can sometimes pick it up for $170 or so, and they don't charge anything beyond that for updates, etc. If you pay $7/month for Celtx, that's $168 in 2 years. So if you use it longer than that, it's cost you more than FD.