+1 on what Wags said. Hex starps save you weight in fabrics and tie out lines and cover you in most situations. Rectangle/squares usually need 3-4 lines per side for a nice taut pitch, diamonds lack year round coverage, but your price range doesn't account for any hex's really. A rectangle or square tarp will take care of you in everything I believe. I'd say atleast 10' x 8' and I actually prefer a 10.5' - 11' ridge line length.
Options could be the pu coated equinox's, guide gear 12' x 12', chinook 12' x 14', hennessy pu coated hex, claytor 10' x 7'(I believe) or save up more because you'll probably end up buying another one soon anyway. In the mean time there's no shame in a wally worl blue tarp. Only shame is not getting out because you don't have a tarp.