I don't disagree that it'd be better to simplify the tax code but the idea that one dollar spend yields seven dollars return doesn't actually work as you're describing.
(this paragraph will assume funding for the IRS is done as a flat tax which is how $43 was arrived at)
Most people might pay $50 per year to fund the agency while a lot of people are paying nothing. Increasing the budget to reduce evasion will lower how much most people pay closer to that theoretical $43 while also increasing revenue overall. Right now it isn't a problem of the common person getting extorted by the government, the issue is that some people are not paying their fair share and increasing funding to the IRS will catch and punish those people while increasing the funding for the government overall.
I am with you on wanting a saner tax system but that isn't really in scope here. Some people are cheating the tax system and it isn't expensive to find and rectify the situation, it isn't even cheap. Tax law enforcement is so lax right now that lowering tax enforcement _costs money_ raising tax enforcement _yields money_ (for a while, there are eventually diminishing returns)
So long as the IRS is taking in more money than it is spending to do so there is absolutely no rational reason not to make sure it can maximize that.