Why would you need to know such a thing?
In most cases, IME, the DM either doesn't particularly care about how you're carrying your money around, sort of like with spell component pouches, or if the DM does care (a) you're high enough level where you have magical storage or (b) you're low enough in level where it's figured there'll be room for it in your gear storage.
But, if you really need to know for some reason, 50gp = 1lb of gold, and
http://crystalkeep.com/d20/rules/DnD3.5Index-Equipment.pdf has sthe storage capacities for backpacks, pouches, sacks, etc. in the last few pages.
IIRC backpacks have a storage capacity of 60lbs, pouches have 10lb capacity, sacks have 60lb capacity, and varying amounts of cubic foot capacity. Sacks and backpacks have pretty much identical statistics, from what I remember.
The ratios between coins and weight is in the trade goods section in the beginning of the equipment section of the PHB/SRD.
Where else that kind of information can be found, I don't know. Bags of Holding/Handy Haversacks list how much they can hold.