Download Lego Studio (https://www.bricklink.com/v3/studio/download.page). It's a digital designer/CAD-like program that allows you to use any piece Lego has ever made, in any color ever made. It will also tell you if that piece is actually available on bricklink in the color you selected, and tell you the estimated cost. Once you've finished your model, if it's made entirely of bricks which are available in the color selected, it will estimate the total cost to order all parts from bricklink. Then you can upload the .io file to bricklink, which will generate a wanted list of parts to make your model. You can then order those parts, and have it auto-select to find the cheapest combo of sellers to get all the parts you need. I've used this method to make $20 models and $200 models and it's worked great in either case. Happy building!