Budget authorization
Like any well-governed organization, our authorization to spend is tied to our budget. See for example the December 10, 2013 budget motion:
Brett made a motion to authorize expenditures of single items up to $200.00 within budget limits and category limits, Ron seconded. CARRIED.
What this means is that we have budget categories and an upper limit of spending in those categories. E.g. $1800 for satellite phones, $100 for advertising, equipment maintenance $3000.
We've preauthorized payments less than $200 for those categories, as long as they don't go over the category limit. Those cheques don't have to go before the board, they can be signed right away by the signing authorities (Administrator, President, Vice-President, Treasurer).
 Exceeding budget
Any payment that will take us beyond the limit for a budget category has to go before the board, even if the amount is less than $500.
Whenever we're about to or projected to exceed the limit of a budget category, the administrator or treasurer should highlight that to board, and a discussion should result of how it will impact our organization.
Most organizations have a difficult decision at that point as they decide whether to go into the red or to cut back on other spending or beef up their fundraising.