qanda,
You need to look closely at what civs you are playing because the cost of some improvements is cut in half for civs like religious, scientific, and military. You can work out the permutations now that you know to look.
I think that some improvements are totally worthless in certain situations and learning to assess these is what graduates you from a Newbie up to a Novice or a Journeyman.
Worthless might better be stated as "a negative asset" since in many case the maintenance cost for an improvement may exceed any added value and will actual reduce the power of your civ.
The best example of "a negative asset" improvement is the courthouse depending on the distance radius that this improve is from your capital city. Less that 4 squares away and there is not enough benefit to pay for the maintenance costs so you lose money. (If you looked at many newbie games you would find courthouses in the capitals and FP cities just sucking away at the treasury). If your city is more than 12 to 15 squares away from the capital city then the courthouse again may not be of any value. There is a narrow range where it has benefit.
I also love people that build SAM batteries in their core cities even though the cities cannot possibly be reached by any attacking aircraft.
For me, I try to save the colleseum to use as a pocket to prebuild units or other improvements, I usually complete 8 or 10 Hospitals, Banks, and Factories in just 1 or 2 turns after the tech for them is available just by using the colleseum to bootstrap up. If you look at my games a turn or two before cavalry, infantry, battleships, and/or tanks you will see almost every city saving up shields in the colesuem to switch over to these valuable items on the quick time.