I am not familiar with WEN, but if it's a no-shampoo method, you can try other cheaper options. If you want to do a no-poo method, read the forums at naturallycurly.com (and you don't have to be curly). You can co-wash (conditioner-only wash) with drugstore conditioners as long as they don't have sulfates or silicones. Suave Naturals run about $1 for a big bottle and smell great. Some of the key items are to remove any silicones first with a clarifying sulfate shampoo and not use silicones afterwards or they will build up. Then, when you wash with conditioner, scrub good with the pads of your fingers for at least a minutes. The friction combined with the water and cleansing properties of conditioner will get your scalp clean enough. Some co-washers use shampoo on occasion.
My DD14 had an oily scalp. I got her the shampoo for oily hair to try to keep it at bay. When I found out about the Curly Girl method, I didn't know if my kids would scrub enough to co-wash, so we went with lo-poo- sulfate free shampoo. Ever since we stopped stripping DD's scalp with the harsh stuff, she hasn't had oily scalp.
We use Loreal's sulfate free line. The new EverCurl smells the best. About $7/bottle. Then the kids use some Suave Naturals as a leave-in. I have co-washed occasionally with Suave Naturals or Tresemme Naturals.