Weighing in a little late, but anyway- One of the problems I've found with low balling to get in the door is that commitees and purchasers at many events like these seem to turn over way too often. That special favor you may have done while one person was "in office" may be completely unknown to his/her replacement, may not matter to them because they've got a friend/cousin/janitor that has got a great Radio Shack PA and will need to have the ridiculous amount you charged last year re-justified.
Referring to Tim Mc's post above; if it truly is a official bid situation, make sure you read the complete bid pack. Gifting, however unintentional and unmalicious the intent, is often frowned upon if discovered and can disqualify your company from this and future bid work with this purchasing entity.