One roll of Gaffer tape - used for other things too so I already had that, so £0.
Foamboard (not paper type) was £5 from one of those end-of-line stockist places.
Ecomodder stickers (now arrived, yay
) ~£10
Money currently saved is running at £20 per tank. Each tank costs me about £62 and I used to get about 430 miles. Now I get about 600 before the panic beeper kicks in. I've also gone from roughly 2 tanks a month to about 2.5 every 2 months.