The only problem suzie1 is that cheap means they won't wear well and could get damaged with frost; much better to go for good quality slabs on special offer.
We have a yard near us where they make garden fences, etc.
and they make their own slabs so if you can find such a place
near you, then they'll be much cheaper. If you need a large
number though, you may have to order and wait for them to be
made.
One other point, be sure not to do buy cheap slabs that are very
smooth. We had some like this once and in the end we took
them up and used them for a summerhouse base because they
proved to be slippery and dangerous when wet.