Built mine from reclaimed bricks & slates from crappy extension I knocked down & left over timbers from new roof 🙂 just had to pay out for some custom made doors. It’s great – not quite a garage but enough space for plenty of bikes.
If you took your time, and used the likes of gumtree and freegle, you could get a whole load of concrete block or bricks, whichever you are using, for a lot cheaper than buying new. Doesn’t really matter if they don’t match if you then render or rough cast.
Buy yourself a second hand cement mixer, you’ll sell it for probably the same as to paid for it post build.