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I am magnetizing an Imperial Knight, and I need to fill in some hollow spaces to offset my magnets. Can anyone recommend a cheap alternative to green stuff for this purpose?

Just to make sure but are you meaning greenstuff bought from GW or kneadatite in general;
https://www.amazon.com/Kneadatite-Yellow-Epoxy-Putty-Green/dp/B00MQ7EHHC

Yeah kneadatite. I was looking at modeling paste or clay, but it may be too brittle for filler.

I would still use green stuff

I don't know if it's a ton cheaper but I use Apoxie Sculpt for my bases, it's a 2 part gray coloured putty.

It works pretty well. Other than that maybe check out hardware stores for some cheap stuff?

Milliput is cheaper, holds magnets well, and can be sanded.
I barely used a pack doing one titan.

Consulted with a friend, and this is what I settled on doing:
1) I superglued a substructure of sprue pieces into the hollow space, tic-tac-toe style.
2) Fill in the top layer with green stuff and place the magnet.
3) After it dries, wet sand with 1500 grit sandpaper.

Seems to be cheap and cut down on any potential weight issues.