You must focus on 2d-fying your objects if possible, so they can slot and fit together, and place multiple parts in the same injection mould... in that way you can reduce the cost of your final product by many times, by thinking of the product from a production-simplicity viewpoint rather than topology performance, and end up with a crazy cheap product. it's sometimes 10 times cheaper to make 2 half moulds that are ultrasound-welded or latched than a full mould.
I highly recommend that you get involved with a 3d printing shop. 1 kilo of plastic costs 1500 rupees and the electric cost is about 50 rupees. You can refine a design over many generations of testing and consideration.
It forces you to learn about plastic melting points, adhesion, topology optimization, using i.e. ABS which is the most common plastic in toys like lego and car-plastics (strong).
If you 3d print with ABS plastic, you can print at 100 percent fill rate. It glues the filament completely-glued to undernearth and neighboring plastic, thereby 70-80% as strong as ABS moulding.
You can study ABS print versus mould strength comparison and apply them to your 3d shape...
You can strengthen an ABS prototype 2x strength by treating it with Acetone Vapor, and painting it with acetone with a brush, where liquid acetone percolates into the part and fuses any voids together. It can make perfectly glassy surfaces. It can acheive strength very near to an injection moulded part.
It depends on the complexity of your part also. Some moulds are 3-4-5 pieces coming in at different angles. Some have only one CNC milled piece of metal.
I bought a RepRap Printer for 22000 rupees. It was 8 hours of great fun to build following a video guide. The resale value two years later is 17000 rupees after printing 7-8 kilos of plastic. It was a very valuable asset for designing and learning plastic.
I also made silicone-detergent cheap silicone moulds for my parts and poured epoxy resine /polyurethane resin in them for fun, but that was a waste of time.