Player's Handbook (pg114) states "This cost is the same for a Small or Medium version of the weapon. A Large version costs twice the listed price." It could then be extrapolated that the costs for sizes larger than Large would double the price again for every size category up you go. It could also be extrapolated that sizes smaller than Small have the cost halved for each size category down you go.
As for weight, Player's Handbook (pg114) states "Halve this number for Small weapons, and double it for Large weapons." Again you can extrapolate this by either doubling or halving the weight for every size category you go up or down (respectively).