I'll pile on and add a third recommendation for Savage Worlds. The rules are not complex but have enough options and depth to keep players and GM engaged. I've recently kicked off a new campaign with this system and I am loving the minimal bookkeeping on the GM side of the screen and the players seem to be happy with it as well. It's very streamlined and supports tactical/map based skirmishes and has a system that can handle large scale battles as well.
As Mr. White mentioned the book is less than $10 and Pinnacle offers a PDF version as well if that's your preference. If you're really wanting to save on materials host the game in Roll20 and you don't even have to spring for minis or maps.
I'd recommend taking a look at Gamma World...the most recent edition from WotC. It's based on a simplified 4th Edition ruleset. It's very easy to get players involved and to learn. Can be as story driven or battle heavy as you'd like. Plus you can play as plasma sword wielding sentient apes, pulse rifle firing birdmen or just a guy made entirely of energy. I enjoyed it.
Games
How to resolve AdBlock issue? 
