300 is cheap. I've seen some prep courses that cost over 1,500.
Honestly this whole process is so expensive, 300 is a drop in the puddle. I spent 600 just on application fees, 375 to send GRE scores, 150+ in FedEx costs, etc.. It all adds up... If you're short on money, which is understandable, I hope you will have enough when the time comes to apply.
As for your question, no a course is not necessary. It depends what kind of person you are. If you cannot force yourself to concentrate, learn from your mistakes, take practice tests, drill vocabulary, practice, practice, practice, then you might need a course. The books have all the info you need. I spent maybe 60 or 70 on books and flashcards, and I studied about 6 hours a day for a month and did very well.
If I had to do it over again, I might try Magoosh online prep course. It's fairly cheap and I've heard very good things. In the end it probably would've cost the same.
You definitely need to get Kaplan's general book since it comes with like 5 practice tests. You can get more practice tests from Princeton Review online by "pretending" that you have their book. You can find the ISBN on Amazon and paste it it, and that gives you 2 more practice tests. The book also has a good formula guide for the math section. Verbal you need to read a lot (NY Times, New Yorker, Politico, scientific articles). I read all the time anyway, so reading comp. was a breeze, but either way you'll need to memorize vocab. I recommend getting Barron's Essential Words for GRE and PR's Word Smart for GRE books. Buy flash cards and go through each book and make flashcards of the words you don't know. Math is just a matter of memorizing the few formulas and hammering it over and over and over again with practice tests.
In terms of books, Princeton Review is great for vocab, Kaplan is decent at both, and Barrons is great for math. There's also a book called Nova GRE for Math book but I found it overly complicated and too difficult. Some people say it's necessary to get an 800, but I got a 750 by practicing Kaplan and Barron's math problems.
Good luck