It depends on what you already have.
Beyond the basics for AG, in order of importance IMO are:
1) Temperature control
2) Yeast starter stuff etc...
Proper temp control = happy yeast => Great beer
Having said that, if you are good at DIY stuff, you can build a stir plate for pretty cheap ($24 for mine, but I had an old hard drive and fan from old computer) add $5 for stir bar and $20 for 2L flask (I use an clear growler)
So you could come in around $50 for all that.
The temp control modules are pretty cheap as well but you would still need an old fridge or chest freezer and you would probably exceed the $80 by the time you are done.
BUT... to me, it is worth the extra $$ if you can afford it. I picked up an old chest freezer at a garage sale for $75, the STC-1000 temp controller was given to me by a buddy who does not brew anymore.
I can ferment an ale or a lager at a precise temperature and then cold crash it AND then serve it all by doing nothing more than adjusting the temp on the controller.
My beer has gotten significantly better and I did not think it was bad before, but just much better in terms of overall taste, crispness.
My first batch using this setup was an Irish Red Ale that I had made 3 times prior. I fermented at 63 degrees for 1 week, then increased to 67 for 2 weeks, cold crashed at 40 for another week.
I poured a glass for a friend that comes over and samples the product all the time and he was so amazed in the difference in taste, he accused me of pouring him a bottle of another brewery's beer. He drank it and followed me to the keezer and had to see it for himself.
MY VOTE is clearly for temperature control. I was making starters and swirling them by hand and the results were no better. I actually use the keezer to cold crash my yeast starters as well.
