Like everyone said they do sell for more but it all depends where you buy it from and if you can get it. Most people are willing to pay 15-50$ each to not look no further.
You may not be able to use these. Would have to see what type of plastic it's made out of. It could possibly leach bpa or other chemicals into the water.
Your gunna want atleast 50-60psi going through your rodi. For proper filtration and less rejection rate. your best bet is to get a booster pump. But that's only if your filters and membrane are good. When's the last time you changed them? What's the tds in and out? Did you flush the membrane?