It's conventional knowledge that if you work on a project yourself instead of hiring a professional, you can save money. Unfortunately, there are times where being a non-professional wastes money if you lack the experience, best-practices or even insider connections for certain supplies. Take a look at a few situations when DIY (Do It Yourself) can be unexpectedly more expensive.
The Fixture You Want May Not Work With Improvised Plumbing
Unless you know the contractor who handled your plumbing, don't assume that you're dealing with standard, professional pipes. For all you know, the previous homeowner could have improvised with some tubing of their own and sealed everything off with putty.
Before committing to any installation, make sure to inspect your current plumbing fixtures. If you can get the model, look it up. Start early on a day when plumbing supply professionals are on the clock so you can take a picture or discuss the fixture, then discuss the plumbing. You may run into a few poorly improvised projects that could make your current plans useless until a professional builds entirely new plumbing.
You don't need to start putting on the new fixture right then and there. In fact, you don't have to buy the new fixture. Too often, a hard-charging do-it-yourselfer will purchase what they think is a standard connection faucet, shower system or outdoor spigot, only to discover that the previous owner has a steel pipe embedded in the wall from some random scrap heap. You may need money to fix that problem before spending it on new attachments.
The Plumbing Work Can Be More Than Expected
Buying things ahead of time isn't the only problem. You can look up guides on how to connect certain pipes, how to get the best performance from your water flow or how to fix certain leaks, but these guides won't teach you the finesse needed to work with certain sensitive materials.
Aside from fragile and complex fixtures, you could be working with old pipes that are falling apart. Rusted pipes can be a health hazard, as pulling the pipes and other infrastructure can maim and infect you.
The theory of just taking out a pipe for replacement becomes complicated if you don't know how to inspect for certain issues. One wrong move (or a series of wrong moves as you work with one wrong pipe) could land you in an expensive rebuilding project that you didn't ask for.
That doesn't mean you can't give it a try, but you need to at least make sure that plumbing supply professionals are on your side. Don't just walk in and ask for parts; call a plumbing supply professional and discuss your project. Ask questions, consider suggestions and take heed of any warnings that seem a bit odd. When you're dealing with plumbing or home remodeling in general, there's quite a few things that can only be learned by experience.
Get in contact with a plumbing fixtures expert to discuss what you need and whether it's more expensive to call a professional or not.