Option one: What will most dramatically improve your life economically? That will almost certainly be some tool that you can use to create a side income stream. (In the third world, it's often a sewing machine.)
Option two: What will most dramatically improve your quality of life? That depends. What inconvenience most annoys you? Could you solve it by buying something that costs less than $50?
My fiance volunteers for PAWS and it is typically a minimum of $350 in fees to leave with a dog. Also, dog food can add up (the dog we recently adopted needs grain-free food due to an allergy) so it's about $750 a year for a 28 lb dog (I would say minimum of $300 for cheaper food). Then you should take your pet in once a year to the vet and get heartworm, flea & tick medicine, etc (another min. $350 a year). And you'll probably want to get your dog a doggy bed, collar, leash, treats, etc. Obviously anyone can cut out or short cut some of these expenses, but this should be the minimum spend expectation in my fiance's opinion.
A change in perspective can be incredibly powerful. And valuable. And I think, anything that can alter the way you see the world and yourself dramatically, is probably the most dramatic way to alter your life.
Yeah, I'd like to know as well
So either a some dumbbells/kettlebell or a good book. You might be able to buy both....
I paid around $60 for a Corsair a couple of years ago. I'm sure slightly less fancy brands or models go for under $50 once in a while.