Change symbols with alt modifer so this has probably been asked, but I have not exactly been able to figure it out. I was just wondering if it is possible to change what the alternate characters are on my keyboard. I have osx 10.9 and use the keyboard on a 15" macbook retina.

For example I never use the ¢ or £ symbols, but frequently use greek letters like η, or λ. It would be nice if I could have all of my frequently used symbols to be mapped to what I want. 

I have googled this and not really found out anything useful.
Not exactly what you're asking for, but I use 

System Preferences - Keyboard - Input Sources


to add Greek &amp; other layouts, and tick the 'Show Input menu in menu bar' option. This results in a pretty menu with keyboard layouts marked by national flag. And, a ctrl-cmd-space shortcut to cycle between them.

Also:

System Preferences - Keyboards - Show Keyboard and Character Viewers in menu bar


for all other typographical symbols