The only one I can think of is Tresemme Naturals. I think its sulfate free. I know at WalMart its under $5. But I can't vouch for how good it is; my sister tried it solely because of price, and took it back because she didn't like it. She ended up buying Loreal Sleek and Shine (which I won't use because the smell is awful for me) at about $8 a bottle, and likes that much better.
I guess it depends on your criteria. If it's solely price, or if price is your number 1 driving factor above everything else, then product reviews aren't going to be as important. If product reviews carry more weight and you're willing to raise your price point by 2 or 3 dollars, you'll find better candidates.
And then, too, are you looking for 100% sulfate free, or modified CG? Because Trader Joe's shampoos are not sulfate free but they are SLS free, and they are less than $3. I rotate TJ's Refresh in my routine and have no problem with it being too harsh, because the sulfate in it is pretty low in the ingredient list, and I only use it every 4th time I shampoo.
My sulfate-free 100% CG-poo cost me around $6.50 at the health food store
I did buy it on sale though. (It's from the Sonoma Soap Company and I love it, it gets my hair squeeky clean and it amazingly gentle.) But still, a great price for a 100% CG poo.
So there are options, but your price point may be low if you want real quality. Quality ingredients do cost money - you get what you pay for.
Some people do like the Tresemme shampoo though, I don't mean to knock it. I just know my sister didn't like it; and her hair is thin, not overly dry, 3a, and has been damaged from too much straightening. Just based on her feedback, I know I probably would not try it.
Don't forget that on drugstore brands - like Loreal, Tresemme, Garnier, etc. you can get coupons, either from the companies' websites, the newspapers, or magazines. That can lower prices that might seem out of your price range. (Coupons are how my sister buys the more expensive Loreal line she uses.) If you have Fresh and Easy markets in your area, they have a customer appreciation program that gives you dollars back for shopping in their store - you can spend your free dollars on shampoos
Other markets may do the same.
And as someone else said, check TJ Maxx, Marshalls, Big Lots, other discounters, they will carry brands that might usually be out of your price range. Maybe not $5, but when you figure out the price per ounce for the biggie sizes, it would probably come down to about the same as $5 a bottle for the smaller size.