Before you start building you need to answer three important questions:
"Pictures. Take lots of pictures. All the pictures."
Find all the reference you can. It all starts with pictures. Take lots of pictures. All the pictures. If you're looking at recreating something from a movie, find the DVD and use your computer and VLC Media Player or MPC-HC to capture stills. If you don't want to buy the DVD, have you checked your local library?
If you're recreating something from a video game, same thing, take a lot of screen captures. Publishers are also getting better and better at providing reference for cosplayers. Two good examples of reference sets for video game characters are Blizzard's reference kits for Overwatch or Heroes of the Storm.
Even if you are not trying to recreate a prop from a very popular universe, check communities like The Replica Prop Forum or a forum dedicated to the universe you're interested in. You would be surprised how many people are likely out there trying to do the same thing you are, and there is power in pooling your efforts together.
At that point, you might have noticed a few things about the prop you're working on, and how functional it can or can't be. For example, movies use different versions of the same prop depending on what it's going to be used for. Close-up shots? That's a "hero prop" with a lot of detail and care put into how good it looks on camera. Action sequence? That dagger in the character's hand probably has the bare minimum decoration so you can identify it, and is likely not made of metal. The amount of detail you want or need in your build is going to have an impact on the finished product, yes, but it will also have an impact on how much time you will need to finish it. That is planning for accuracy.
This will also help you figure out how long your build needs to last - maybe from just a few days for a convention costume, to several years and more for a collector piece - and how long you have to build your replica. Are you prepping for a local convention in 2 weeks or for next year's Comic Con? Are you going to gift your friend for the holidays in a few months or for their birthday next year? That is planning for time.
Now that you know what kind of replica you want to build, how long it needs to last and how long you have to build it, you know everything you need to start looking into the materials you will need and where to purchase them. At that point it's not just about buying cheap materials for your build, but buying the right materials for the build you have envisioned. This is planning for budget.
By then, you're ready to order your supplies and start building when you get them: if you can afford it, order about a third more of everything than you planned for. It will give you wiggle either a) for experimenting or b) for messing up during the build process. And by spending a little more money right off the bat, you might actually save yourself either money (you might get discounts from vendors for larger quantities, and save yourselves repeat shipping fees), time, or both.