That’s exactly what enterprise sales is. Call for pricing, tell the absolute upper limit you can spend, and pay.

Is there a guide to negotiating enterprise software purchases somewhere that doesn't suck? I feel like I am always overpaying.

Figure out what you need, get the final quote, and go silent.

Eventually they’ll come back and have some impressive sounding exec reply saying they need 20% less to get it in the budget.

because you always end up paying... from time to time do the exercise of not purchasing but finding out their lower limit

A nicer way to frame it is: “value based pricing”.

hire a salesperson to give them fake sales numbers and low ball them so you pay less?