MtBakerVapor has some really cheap juice (~40$ for a 236mL). I always keep some bottles from them in my stash because it's just so gosh-darn inexpensive.
My go-to's from them are Hawk Sauce -- kind of tastes like a blue raspberry sour straw in my opinion -- and Thug Juice -- it's watermelon and a lot of menthol. But the juice from them is so cheap that you can scoop a bunch of 15mL's just to figure out what you like and then re-buy en masse. They also do free samples with your order.