It depends on what you're looking for. Second hand prices fluctuate, but they consistently go for 50-70% of retail price most of the time. Some
FLGS and online retailers offer new
GW stuff at a 20-25% discount, if you're looking for new models to assemble and paint without the hassle of stripping them.
A drop pod, for example, goes for £27.50 as the latest price increase hit two days ago. A second hand one for £15 is not a bad deal, but not that uncommon either.
Another cheap way to get models, especially for
SM players, would be through the starter boxes.
Getting the
SM half of the new starter box (depicted under spoiler below, it should arrive in July alongside 9th edition) could be a good and relatively cheap way to get some new models.
BA should be able to use them, according to the latest info we have from
GW.
Aside from that, the current starter box (Dark Imperium) is going out of print soon but it also has a few
SM models that could be useful.
Shoulder pads may be an issue with these boxes though, as the models are single-pose and have a plain pad already attached to the arm.