Everything seems expensive these days. I think the NT membership is worth it if you visit 5 or 6 places and we did 7 on this trip, but then the prices vary. Other places may be English Heritage or independent. It may be worth checking out for special deals on tickets. It worked out well for us as Rievaulx was taking NT members for now. For those with kids it can soon add up even with family tickets. Of course there are free things to do like country walks or exploring cities. You can find some free online city guides out there. Some places like churches just take donations.