Cheaper alternative for RTV mold release spray???

What are you casting? If it isn't something really chemically aggressive, why use mould release at all? I never bother with it in RTV moulds.
If you're casting, say, urethane resin into a silicone mold, and the object has very smooth surfaces, you don't need to use mold release spray all the time. With very smooth planes, if you load up the mold release, it'll just show up as imperfections on the cast anyway. While mold release can help increase the performance life of the inner surface of your mold, it's not necessarily required every time.

If you're casting more complex models, or very rough surface, though, you do want to make sure your mold releases your cast. A can of mold release nearby is handy for those times.
Well, there are other products, like sonite wax, but you don't want to use common household chemicals as a substitute. Mold releases like Universal or EaseRelease 200 contain specifically designed mixes, but other chemicals may harm your model or cause inhibition in the mold rubber. Nothing is worse than having to try to clean uncured rubber from your original. For this case, I'd advise you to stick with the chemicals that are designed to work with silicone in its uncured state, like the ones I mentioned.
Actually I meant when pouring the RTV rubber onto the master to make the mold.

I generally just give the original part a coat or two of carnauba car wax and buff it out...

If I use a wood, cardboard (non coated cardboard aka unglossy) or MDF mold box I generally brush on a coat of Vaseline and hit it with the heat gun while buffing it off, it soaks in giving a superb release surface... I find this very helpfull on MDF where I have silicone soak into unsealed MDF and bind, a thin coat of Vaseline melted into it solves that problem and it's NOT even greasy to the touch afterwords...
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