The fundamental question is what are you looking for, a pair of speakers, or a long-term past time? That's part of what lennon_68's poking at.
DIY speaker design is a hobby, so it takes a lot of time, and you spend far more money on your first one than required to just build a speaker. But, it's a hobby with levels of involvement - you don't have to dive into the deep end. The question becomes how deep you want to go?
- can you assemble a kit that only needs a screwdriver?
- can you build a box that looks nice enough that you'd want it in your living space?
- can you solder comonents into a crossover from a circuit diagram?
Kits are available at each level, from screwdriver-only assembly to a bag pf parts and instructions. Clearly the former has little associated cost beyond a pricey kit, while you'll need some woodworking skills and tools to build a box.
Note that I haven't mentioned crossover design, as that's another layer of associated cost (measurement rig) and a lot of learning even if you have a basic electronics background. However it one that's easily bypassed due to the availability of lots of "proven designs" that have been built by multiple users. HEre's the master list.
And for parts, PE is one source, Mensicus, Madisound are others, and that just a start.