I want to make a 2-part mould roughly covering 3.5sqmtrs but the overall cost is quite large.

Would it be possible to use a cheaper alternative to the Uni-mould Tooling resin (which appears to be the most expensive part) such as a cheap Polyester resin to bond all the layers of chopped strand mat? Or is there another cheaper alternative to the uni-mould system that would give adequate results?

I don't intend on pulling hundreds of releases, at most I would say 5 in total?

I use regular polyester almost all the time.  You just need to watch it for shrinkage.  Build it up a layer at a time and let it cure before adding the next. I found this stops any shrink and removes the need for a filled resin.  If you're just after a few pulls you can use regular gel coat too. It's just softer than tooling gel so wears poorly.