Originally posted by Mr Spice
I agree that caravans are a better choice. But if (big if) we should happen to get into a situation where we have to buy some wonder or loose it, we could use a large treasury. 
In short, I think we should be careful not to waste our money.
But don't you see, spending 56 or however many gold to complete a caravan now could actually save us money should a wonder rush hit us. That is a travelling 50 shields that we've created, and those 50 would cost a fair amount more if we have to purchase those shields as a wonder-completing package. It is much more expensive to buy wonder shields than caravan shields.
The only way we lose out is:
1. The caravan cannot get TO the intended city in time to add its shields before another civ completes it (therefore sooner can be better than later).
2. By spending money on caravans we are unable to meet a specific military threat to the nation by being unable to rush build units (do we need 850 gold for this?)
Edit: 3rd way: We would win every wonder race we wish without any rush building. Seems unlikely.
Consider this: We could very well get the message that the Chinese (for example) have nearly finished Leo's Workshop. We might find that it would take 800 gold to finish it in the city with the most shields but we only have 600. If we then went back about 3 turns or so, we may realize that to build the needed 5 caravans, it would have cost us 500 gold with incremental rushing from cities building caravans or those that could switch from units. Indeed, a 300 gold difference could be the case - perhaps even more.
If we rush enough to have 6+ on hand, then we need not fear the last minute wonder buyout.
I think that rushing some caravans could also help us right at the moment. By finishing Cope's ASAP (assuming we get Astronomy ASAP as well), we could force the other civs to abandon their projects and have to start a different wonder from scratch when the techs are discovered. I know we have a fair number of caravans now, but we may be using some of them as actual trade caravans.
With our 850+ gold, we could probably get to 12 food caravans over the next 10 turns - not even taking into account the gold that will still be added via our surplus. This could just about guarantee us the next two wonders without even having to worry. We could get Cope's (when we have Astronomy) and then build Sun Tzu's right away to keep the civs that have feudalism from changing to this one as well. I have to doubt that any other civ has Theology at this point - so they will have to abandon projects and lose shields. Plus, we'll have these wonders sooner, can make use of them earlier and get a jump on the next wonder.
Man, did I just ramble or what? Sorry about that. I'm just not used to playing with such a large surplus so early in the game - I am usually rushing just about anything I can while leaving a few hundred for an emergency. Mostly caravans - both kinds.
By the way, this becomes an even more emphatic point if we decide to keep our Tax/Sci/Lux rate where it is or even increase taxes. In either case we will be earning a very nice surplus indeed.