Check out this print maxi at £26 from www.boohoo.com
From Asos.com try something plain and simple, £28, comes in Cream, Apricot, Turquoise, that is easy to dress up or down:
Both boohoo.com and asos.com have a wide selection of maxi dresses to choose from all at reasonable prices, at both websites have a function where you can see the dress live on a catwalk so you can see exactly how the dress looks on, whether you like the cut etc before you buy it.