Per Basic Fantasy, gold coins are 5 per cubic inch. We assume all coins are about the same size. Gold coins in our realm are more like ancient Roman coins, 50 coins to a pound. Platinum is only slightly heavier than gold. Silver, electrum and copper are about half as dense. Exchange rates between coins are generally fixed.
Exchange Rates
1 platinum piece (pp) = 5 gold pieces (gp)
1 gold piece (gp) = 10 silver pieces (sp)
1 electrum piece (ep) = 5 silver pieces (sp)
1 silver piece (sp) = 10 copper pieces (cp)
Convert to Gold
1 PP = 5 GP
10 SP = 1 GP
2 EP = 1 GP
100 CP = 1 GP
Weight and Volume
50 GP/PP = 1 pound
100 SP/EP/CP = 1 pound
5 coins = 1 cubic inch
1 cubic inch = 0.0043 gallons