Can alternative materials replace sheet balsa? The answer is yes, and it has in some cases, but what about weight?
Some free flight sheet balsa models use sheet balsa, and balsa has a density of between 5 and 10 lbs per cubic ft. ( 88 - 160 kg per cu metre)
A sheet balsa wing of 12 inches span and 3 inches wide ( 30 cm by 7.5 cm ) will weigh about 1.6 g and can be replaced by three bamboo spars and covered with 80 gsm paper. The paper will weigh 0.0225 sq metres weighing 1.8 g. What will the bamboo spars weigh?
Twice or more the density of balsa, so say Bamboo 300 - 400 kg/ Cu metre.
Volume of the spars : 30 cm x 1mm x 1 mm = 0.0000003 cu metres, and will weigh 0.36 g
So we have: balsa wing weight: 1.6 g
80 gsm paper weight for the wing area: 1.8g
Tissue paper weight for the wing area : 0.9g
40 gsm tissue paper wing with bamboo spars 1.26g
80 gsm paper with wing bamboo spars 2.16g 35% heavier
Cardboard is about 7 times the density of balsa, so using cardboard sheet and cutting out 1/7 of the area or more and covering with the 80gsm or tissue paper will result in a fairly light wing.
For sheet models such as those at Paul Bradley's Model Airplane Plans Page
http://www.parmodels.com/Plans/jigtime.htm using cardboard and tissue is a possibility.
Posted this in my scale free flight thread also, but this is a simplified version
These links will take you to the data :
References
Densities
Balsa 88 kg/ Cu metre
Bamboo 300 - 400 kg/ Cu metre
Cardboard 680 kg/ Cu metre
Paper weights per (grams per sq m)
10–35 gsm tissue paper
35–70 gsm lighter textweight
70–100 gsm medium textweight
http://www.wisegeek.com/what-is-tissue-paper.htm
http://www.simetric.co.uk/si_wood.htm
http://www.pelaero.com.au/Density%20chart.htm
http://www.cleavebooks.co.uk/scol/ccdens.htm