Hello there,
To add a little new information here...
I would think that this should be handled just like any other cost analysis with associated future projections.
Since you have an estimate of the gains and the costs, you should be able to answer the question, "Is it worth it", with little or no problem.
The way it goes is just like anything else, where you purchase a product and it gives you a gain, and that gain is measured in dollars per unit time. You know this so you can project the "break even" point in time, where in the future the new product (the cap) pays for itself.
Once you get past this point of course then it is all profit, provided there is no maintenance on the product that would have to be factored in also.
So if you gain 5 percent efficiency and that results in 5 dollars extra per month and the cap costs 10 dollars, then the cap pays for itself in 2 months.
That's the right way to rationalize the cost benefits. If you have a question on efficiency vs cap size, it is probably best to experiment. Try a reasonably sized cap and see what you get over a couple months, then size up again if it looks promising.
I was speaking in terms of dollars (USD) but any type of currency would be basically the same.