Hello and welcome to the forums!

This is a good question that doesn't apply only to small libraries. I can't give you any firtshand knowledge, but a [url http://www.google.com/search?hl=en&q=%22friends+of+the+library%22+materials+budget&btnG=Google+Search ]quick search[/url] informed me that many libraries get help with materials budgets from their Friends.
[url http://www.yorkcountyva.org/index.cfm?action=c10&id=1584 ]Friends of the York County Public Library[/url] in VA call themselves "
Enhancers who raise money for library materials, equipment, facilities and programs beyond the level of the library's tax-supported budget."
Take a look at Table 1 in this [url http://www.libraryjournal.com/article/CA6298437.html ]Library Journal article[/url]. It has some budget figures for a range of population sizes.
Can anyone give some ballpark figures for budget amounts?