Inexpensive, Lightweight Core for Interior Uses?

I purchased sets of plans four years ago, for both the DE25 and the LB26 from Bateau.com. Using the building instructions of both plans, I am going to build something with a superstructure that resembles the look of a Thames 25' saloon launch.

http://www.peterfreebody.com/downloads/Classic_Boat_Issue32_1991.pdf

The LB26 plans recommend foam core and as the DE25 is the easier hull to construct, I shall marry them together. This is going to be a rivercraft only, so ultimate strength is not necessary. The boat would be so light, only a 15 hp 4 stroke outboard would be required. 2 litres per hour @ 8 kph.

Knauf Polyfoam is the same specification as Finnfoam, so I have done some costings for UK construction. The DE25 plans call for 28 sheets of 10 mm and 9 sheets of 12 mm BS1088.The 12 mm 5 ply sheets are available @ £43-32 per sheet but it would have to be Robbins Elite 9 mm 5 ply @ £65-55 per sheet, costing a total of £2225. Dat's alotta dosh!

http://www.robbins.co.uk/pdf/Marine_Pricelist.pdf

Using Polyfoam, 40 mm hull bottom @ £10-51 per 1200 mm x 450 mm sheet and 35 mm hull sides @ £9-27 per 1200 mm x 450 mm sheets, the DE25 plans would require 24 sheets each of both thicknesses, about £480-00. VAT of 20% is ignored for now.

Foam core requires more epoxy resin and glass cloth, but using Polyfoam would save £1745, which buys a considerable amount of epoxy resin and glass cloth, over and above the 30 gallons etc., that the DE25 plans originally call for:

http://www.bateau.com/studyplans/DE25Cockpit_study.htm?prod=DE25Cockpit

Money, money money,
I must save some money,
I'm a poor man now!