rupee value is 8-byte integer:
struct StructGold
{
__int64 m_nGold;
};
max rupee is constant variable stored in GS code (in fact there are 99 of them, MAX_GOLD_FOR_INVENTORY_00 - MAX_GOLD_FOR_INVENTORY_98 in assembly info) and can be changed with simple HEX editor, here is its standard hexadecimal value: 00 E4 0B 54 02 00 00 00.
you have to change such byte blocks to anything you want, for example if you need 1kkkk you have to change it to this: 00 10 A5 D4 E8 00 00 00.
if you dont know which HEX editor to use, i recommend XVI32 (google it).
btw, if you tired searching for a correct value using assembly info and disassembler - there are no bugs found if you set all the values from 10kkk to another in GS so for the first time you could use Replace All option.