that board is a 9'8 - spar and ribs 4mm exterior ply about a half sheet 600x2400 so about £8. I used 2 planks of paulownia (google rooted origin, newquay) - these were about £30 each (Christian did us a bit of a good deal as me and a mate waited a few weeks for stock so gave us the old price - current prices on the rooted origin site. I organised my own collection and delivery through interparcel, so this as another £20). for the rails I used a recycled joist from a local 1920's beach house renovation = free. that works out just a sniff under 90 quid for the wood. I've used pine before, and using 4mm ply for the bottom saves a heap of time and a bit of cash. however ply standard sheet is 2400mm.
add another tenner or so for glue, tenner for finbox, then its resin (3litre poly kit £25 and 1 litre RR epoxy £20) and glass after that - 4 metres wide and 4 metres narrow (£25??). I kind of lost the plot a little with the resin job and lammed a proper goth looking mexican blanket in polyester (pigments £20) then finished up in epoxy cos I hate sanding poly. like the sanded matt finish too. All in - a couple of hundred? My mate did a 7'6 balsa board and i figure it worked out similar cost using ebay balsa. prefer paulownia - nice and light but not as soft. I'm on with a magic carpet that will be ply bottom and paulownia top. aiming for 7kg all finished..
hope that helps - woodies are great, the investment is in the time rather than the materials when you make them.