I won't dare try to answer this question, because the issues involved are more complicated than I can handle. I will say, however, that your question presupposes that the only (or main, or most valuable) reason for public education is to enable and encourage contributions to society. I don't think that is correct. One very important project of education is negative--it helps us to prevent certain social ills and other things that are far less costly to educate away than to deal with later.

Any very complete answer to your question would require the following:

(1) A complete enumeration of all of the goals public education is to serve.
(2) A prioritization of the list accomplied in (1).
(3) An reliable assessment of the financial costs involved in achieving (and in failing to achieve) each item on the list of priorities.
(4) A reliable assessment of the social costs in achieving (and in failing to achieve) each item on the list of priorities.
(5) Because we may well find that we can affordably achieve a lower goal, and not be able to afford a higher goal, we would need to formulate a set of principles by which we can reasonably not pursue an expensive higher priority, in favor of pursuing a less expensive lower priority.
(6) A just and equitable way for government to decide how much money to raise (via taxation, for example) and how much to spend on these priorities, balanced against others.

And probably a lot more, none of which you or I can provide at the moment.

There are other questions, as well--and not just philosophical ones. For example, in the case of gifted children, is it really true that greater resources spent will lead to equivalent gains in their later contributions--or do gifted kids generally find their ways without lots of extra help? Unless we can know what the real costs and benefits are, in other words, it is prudent to try to broaden our conception of our social goals for education before we come to fast (and likely dirty) decisions about how much better we could be doing than we are now.