I have a flat, continious rocker available for $70 plus shipping. The DU stuff is great, just more expensive - about $120-130 depending upon retailer.
As for the debate between concave and flat. It is a matter of personal preferance. Slider riders like flats for predictability. Concaves cut better since the topography channels water and creates low-pressure areas. It is easier to cut without proper technique. I prefer to teach with flats so proper edging technique is learned and then put the more experienced rider on the concave.
Put out $70 now. Learn to ride and demo more boards over the next season or two. Then buy what your style needs - it is personal preference after all. email@example.com