I am also new to this sport (got my wing october last year) this might not all be true but just my thoughts.

Wing: Many new Wings are coming out this year, so I have already seen cheaper prices for last years gear.
AND get in touch with your shop, I asked for a discount for the bundle of wing, boom, leash --> got 100 Euros knocked off.

Board: One idea I had was to cut a bit off my old windsurfboard. Drill 4 holes and then attach the foil. Maybe an additional board to make it stiffer. Certainly not great looking but has been done (at least I found some pictures of that somewhere). I also have the idea that I could build a board myself, wooden skeleton with something floaty inside.

Foils: sorry no idea about that, but they are around longer than the wings so you could maybe find some used ones.
AND if you search for "build a foil" you can find a kit for a DIY foil.

But you are right - learning windsurfing would be cheaper
Wing 600 - 700$
Board 0 - 100$
Foil DIY 450 $

1250 $ ?