Aluminium then. you can find cheap ass frames on Alibaba, you can order one as sample.
In carbon, to be significantly cheaper than velobuild is asking for trouble. We know velobuild is safe, but they are running a tight ship. they dont really do discounts, they dont really do marketing, they are a lean operator, they clearly compete on price.
If you buy the cheapest carbon frame you find on aliexpress, there's so much that can go wrong, the risk of time & energy you may have to waste trying to get shit to work...
China will absolutely sell you trash if you ask for it.