I don't know the answer off the top of my head to this, so this is how I found the file
1) Using a good quality search tool (I use Agent Ransack), search all the game xml files for "available to spend" - this gives one file CIV5GameTextInfos_WorldView.xml
2) Open this file, search for the text and find the associated TXT_KEY (it's TXT_KEY_TP_AVAILABLE_GOLD)
3) Now search all the lua files for this TXT_KEY, it's in TopPanel.lua
4) I have 5 of these, so I open the one from Expansion2 and search for the key, in my copy it's on line 543 and lines 547 to 561 is the code that generates the green text, so you'll need to make a mod that replaces TopPanel.lua with one of your own and add your required changes.
It should be noted that changing TopPanel.lua is not recommended if you're going to redistribute your mod as it will conflict with things like EUI and VP. It would be better to replace just the GoldTipHandler, but you'll need to understand UI tooltip processing to implement that.