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 $ ?

Windsurf stuff (board, mast, sail, boom, extention, harness) will be way more expensive.

Sorry I meant getting 2nd hand windsurf gear is cheaper as there just is not so much 2nd hand wingsurf gear around (yet).
But you are right with the new windsurf and kite gear.