Dimitros,
You are not going to save money. Only do it if you think building is fun.
Read greenlight surfboard supply guide on line.
I build boards for fun and sell a few to locals to pay for my learning.
Have a Fb group called TW Surfboards. Ask to join. You can see my work and some steps.
Best way to get success is to buy a surfboard blank ( PU foam). Shape it, add fin boxes ( difficult step) glass it, coat/sand it, finish coat. Use epoxy.
Minimum material cost in Europe, if you don't have supply shop in your area is 300€.
Besides mastering the skills to shape, glass, add boxes, and finish work without sand-through, you have to have a shop, controlled temperature, safety equipment ( carbon filter mask etc), some tools and shaping stands. Minimum is sure-form, electric router, electric polisher ( rpm 1500-5000) with surfboard sanding pads. It's possible without some of this, but it's going to be really difficult.
So it's an investment in tools, space and time. You have to be in maker mindset.
I love it. Fantastic to make stuff and then use it.
After 10-20 boards you might get this: