It WILL cost as much as buying new speakers, but it could be a fun project. The problem with designing a x-over is, it isn't that simple if your goal is a great sounding speaker. The drivers need to be chosen together to work well and picking an off the shelf x-over isn't the way to get the drivers to work well together. If you want to experiment with building a speaker, I'd start with a kit rather than trying to rebuild something like this.