sealing sandstone

We got some sandstone done in our back garden and are thinking of getting it sealed as there is some moss or algae starting to appear at some points.We rang the contractor and asked about sealing and he said you could just hose it down every couple of weeks or just get it sealed.we have 30 sq.metres to cover and a supplier recomended Drytreat impregnating sealer but at €75 a litre and we need 3 of them is there a cheaper alternative or else pay the money and be sure it will do the job

should have been sealed immediately, some damage has been done, lithofin sealer is best, at tile shops, 45e approx, make sure dry and clean as possible before application