Square 1, from a few years beyond.

This is all data work, and the iron triangle that I look at with data work is fast, cheap, scalable.  You're (presumably) trying to get to square 1 of visibility of your spend.  3rd party tooling vendors can get you to square 1 pretty fast but I haven't seen one that I'd consider cheap yet.

Once you get to square 1 though, you are going to see squares 2 and 3 and 4, and that is where I don't feel any 3rd party tool can provide for real scale.  What I mean by scale is connecting it to the other data you will find lying around your org that will enrich the cost data and allow you to take the practice far beyond square 1.

Some more mature tooling vendors will point out that you can import custom datasets in their platform and essentially turn their platform into your personal data warehouse, but consider the time you'll spend building all of that into someone else's platform.  Think of the vendor lock in, and how much those add ons cost.  Also, the speed that you paid for up front is long gone by that point.

Conversely, building your own visibility takes time up front but beyond square 1 the velocity can be so much faster.

So really it's a choice between going fast only to go slow later, or going slow to go fast later.  Don't discount the importance of optics in the early stages of the practice. You have to deliver some wins early in order to build trust and gain more buy-in. This is just a POV from a few years past where you are now.

Even if you buy a tool to deliver some wins early, only sign a 1 year contract and never stop thinking about how you might replace that tool when you need to. Never stop improving the underlying data quality. Never lean on a product feature that lets you shortcut improving the data quality - tagging, specifically.

Building your own may be a luxury some of us can't afford at all times, but I'm convinced it's the only way forward for most companies.