I agree - but I do have another solution... Don't bother.
Why? Well the law is pretty sensible... So what if you guys live happily ever after - great - no problem. What if it lasts 2-3 years? Well the law would consider it a short relationship and each party takes out what they put in... So no need for an agreement - Great.
What if it lasts longer and you don't have an agreement - say 10-15 years? Well then you're likely to see half your stuff go to the other person and vice versa... Bugger.
What if it lasts longer and you have an agreement but a substantial event has happened in that 15 years - you guys have a kid, or you buy property together or just staying together for that length of time could give grounds for a court to dismiss the agreement, especially if there is any reason for a magistrate to consider it to be unfair?
All of these things would see the validity of the agreement as questionable.
And...
What if it lasts 15-20 years and you have a legally binding agreement? Well if she wants to challenge it, she can. She might win, she might not, but you're still gonna be throwing money at a solicitor.
Now like Hayder said, solicitors are worried about getting sued in the event a court decides to set aside the pre-nup... That kind of hints to me that solicitor's / the legal system don't see these things as water tight. So why bother?
Now as far as making it cheaper. Well for the thing to be close to water tight - both parties must get independent legal advice (can anyone confirm this for me?) So nope - you're gonna have to pay solicitors / barristers and anything you write up on a piece of paper and sign is toilet paper, so the only way to get something close to watertight is to get solicitors involved, so I don't see a cheaper alternative.