The usual explanation I get for expressions such as "How much did you want to spend, sir?" is that the use of the past tense produces a distance between the present reality and the question, thus making things more polite.
The thing is I've found this other example "I wondered if you were free this evening".
It makes me wonder if the origin of the past tense for politeness is an archaic subjunctive use, such as in sentences as "I wish he were here". In the end, it is not far from what the subjunctive is meant to do. It gives an idea of distance rather than the straight-forwardness of the indicative.
I believe English might do that in some invisible instances as this example. The subjunctive mood is sort of there, even if it is by playing with indicative tenses. The same distancing happens when using modal verbs in certain ways too. "That will be £1.65, please.", for instance.
Of course, apart from curiosity, this is a totally useless notion to a native speaker (thus it is not explained in their grammars). But for pretty much any European, or basically anyone with a first language that uses the subjunctive explicitly, having a look at this might be rather useful when trying to grasp the meaning of many English structures. Both modals and apparently random changes of tenses can be hard to understand otherwise.
What do you think? Speaking a language that uses the subjunctive explicitly, Spanish, I would find it very interesting if I get proof for my guess.
It would also make way more interesting expressions as "I was wondering..." with a double distancing meaning.
Note on conditionals vs subjunctives, replying to Benjamin Harmann's answer:
Conditionals are in fact related to subjunctives. Let's look at Spanish, so we have a common point of reference. We've got a conjugation for "indicative conditional" and another one (2 actually) for imperfect subjunctive, also called "subjunctive conditional". They mean the same, although the subjunctive one is slightly more hypothetical and indirect (therefore more polite or educated).
In "If I were/was you" the conditional is marked by if, so we all agree "were" gives a subjunctive mood (even most English grammarians). "If I was you" would be a syllogistic argument that means that I equalled you in the past, quite literally. In Spanish we would do that with "fui" or "era", then we could imply conditional in the verb by using "sería" and subjunctive "fuese" or "fuera". The thing is that the use of the conditional verb in Spanish wouldn't sound right in this case; so the verb would be essentially subjunctive keeping its conditional meaning.
Conditionals can also be expressed without if (or equivalents) thanks to the conjugational nature of the language. This is more easily seen when we use modal verbs. For "I'd like to see you tomorrow" You could say "Querría (conditional) verte mañana" or "Quisiera (subjunctive) verte mañana". Querría here is purely conditional (although as there is no if...then sort of structure, the meaning is essentially the same as the subjunctive, the latter having the aforementioned more polite nuance in meaning). I think English does this sort of thing all the time.
Another relation between conditionals and subjunctives is that they both relate to politeness intrinsically. I don't think this happens in all languages, but there is definitely a purely logical, and universal, nature to it. So, even when English uses the conditional for politeness, by comparison, you can see how this can be sort of the same as the use of the subjunctive. The mood and meaning are basically identical.
Particularly interesting situation: (EDIT)
She looks as if/though/like she is rich. (Perhaps she is.)
He talks as if/though/like he was rich. (But he is definitely not.)
Formal American English: He talks as if he were rich.
In the end, English evolved from the use of the subjunctive, as all related languages. But its ghost, in meaning and use, might be very alive indeed. It feels like many English structures are the remains of a more logically organized, older grammar. In order to understand the logic behind English as a foreign learner, comparisons with other similar languages might be helpful.
And as I think this might have to do with the use of tenses and modals, I'd like to add:
"Contrariwise "continued Tweedledee, "if it was so, it might be; and if it were so, it would be, but as it isn't, it ain't. That's logic." LEWIS CARROLL. Through the Looking Glass
Interesting previous comments:
In Italian, which has a subjunctive mood like Spanish, there are still common expressions like "Mi chiedevo se..." or "Mi stavo chiedendo se..." (I was wondering if...) where the subjunctive is not used, but instead imperfect past tenses are used in the indicative mood.
English has a subjunctive by the way. "We require that he use subjunctive." Not uses.
The comment on Italian doing exactly that is quite interesting, though. In Spanish, I only know of the exact opposite. The subjunctive is used as past by journalists sometimes. "La que fuera ministra de Sanidad." Instead of "La que fue/había sido ministra de Sanidad"