If money is really that big of an issue then go to St Andrews. But honestly to decline Cambridge at any cost for St Andrews, it's honestly settling for second, third, even fourth or fifther or tenth best. Yes you can justify that you saved a lot of money and perhaps if you excel at your studies you can do postgrad at Cambridge or a more prestigious university (than St Andrews) one day. But then you will feel was it all worth it?
Also St Andrews whilst a charming place for a visit can get claustrophobic and tedious a place after a while and you will tire and get bored of it. Yes the university has history and tradition and at the undergraduate level has some prestige but it really isn't comparable to a place like Cambridge which excels pretty much far more than St Andrews and most other universities in the world in what it has to offer.
But you will be saving money which you can use to have fun so if that means a lot then St Andrews is waiting for you.