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Ghostly Prison only prevents creatures from attacking you. You are not your planeswalker, so attacking your planeswalker doesn't get a tax at all.

All players are responsible for maintaing the gamestate. Your opponent doesn't get to activate mana abilities when declaring attackers if there is nothing to pay mana for. Of course, he is free to activate mana abilities after attackers are declared. So if he said something along the lines of "My creature attacks Elpeth and I pay to pay for Ghostly Prison." then that's an illegal action and you have to remind him and the game rewinds. If it's clear, that that is what he's doing, you have to remind him and the game rewinds. If he seems to be taking a shortcut, then you don't have to remind him, but he would have to announce the amount and quality of mana in his mana pool afterwards.

this is the kind of thing that you would need a judge to rule, and they may rule differently, especially according to how things were worded and how much is communicated to the judge. I am not yet a judge, but I have played for eight years and am often the go-to guy for my peers. there might be some discussion in judge circles about rulings regarding players' intentions contradicting their game actions, but you asked for oppinions so I'm assuming you'd rather hear from me than no one.

The things that I believe will affect the ruling are:
whether or not he indicates his intention to attack your planeswalker
if and how he indicates why he's tapping mana
and the timing of these two declarations of intention.

I'll assume nothing else has happened since then. the more things that happen, the harder it is to allow rewind. if enough happens, I would not allow rewind, and would rule appropriately if the game state needs to be legal.

if he makes indication that he's using mana for prison, he's making indication that he's attacking you. if this is at the same time as he indicates he's attacking elspeth, I would rule that this is an illegal action and allow him to untap and declare attackers again appropriately. if his declaration to attack elspeth is seperate, however, I would rule that he is attacking you instead.

if he doesn't indicate that he is tapping mana for ghostly prison, I wouldn't always assume he has made indication to attack elspeth. it is legal to float mana after all, despite not being a common habit.
if in this case he doesn't indicate attacking elspeth when he attacks tho, I would say by tapping the appropriate mana his game actions implied attacking you, and would rule that he's attacking you. if he instead indicates attacking elspeth, I would tell him he has mana available in his mana pool, rule that he's attacking elspeth, but not let him untap lands.

if he says something along the line of, "I tap two mana to attack elspeth" I would allow him to attack elspeth and untap his lands.
similarly, if he declares he wants to attack elspeth any time before tapping mana for ghostly prison, I would let him untap his lands.