Normally the cheapest pleco would be the common pleco that in time will be around a foot or so in length. People go to a petstore and ask for a algae eater and the seller goes straight to the common pleco not warning the customer how big they can get and aggressive. Some good sellers will warn or give recommendations and suit the customer better. Such as a non aggressive tank can get ottos, or a bristlenose pleco to suit medium sized tanks.
Different types of food, and also brine shrimp and blood worms, frequent water changes, adequate tank size, and any fish will grow to its fullest and even more.