Even if us tax payers made 10mm $ on the mueller investigation, i personally would still want to see the extent of russian interference.
We should use such investigation to learn how foreign powers are trying to undermine our country and how we can protect current and future americans from falling victim. It is money better well spent than say, protecting against a fabricated emergency.
Sides, Trump says he's exonerated, he's got nothing to hide, let the public understand all the ways that Russia isn't an ally.
Noting a lot of the interference began under the Obama admin (as confessed to by Brennan) and they did absolutely nothing about it.