I'm getting these for my DS who will be 4 at Christmas time, thanks to mumsnet recommendations! They're currently £30 on amazon for the original set, but I can see a few other brand versions that are cheaper.
Are magformers the best? I'm only a bit hesitant because DS can be a bit hit and miss with essentially any toys that don't have wheels (the only reason I'm not getting him the magformers set with wheels is because my DM has already bought him a junior knex "wheels" set!) and it's quite a bit to spend if he doesn't touch it.