Anyone have tips on finding the best GRE prep course on a tight budget?

I just started looking into GRE prep stuff because my job wants me to get a master's eventually. Thing is, most of these courses seem pretty expensive and my company isn't covering it. Are there any good prep courses that aren't crazy pricey? Or maybe free resources that are actually useful? Or are some of the pricey ones so amazing that I should splurge on one of them? I keep hearing good things about Menlo Coaching's prep, anyone used them? Would love some advice from people who have been in the same sitch.