The Breakeven volume must factor in how much you will spend on the marketing of the event/ promotion. Simply divide your Fixed Costs (total marketing expenditure) by the Gross Margin % of your offer:
Total Promotional Expenditure
Gross Margin % of the offer
= $1250 (of Sales must be achieved to breakeven.)
This is a shortcut method and not 100% accurate, but close enough to be in the ball park. (This presumes you know the total cost of the promotion, and that these costs are all fixed.)
You should remember that a promotion does not ONLY return you the short term sales, but you may be gaining a few new customers who continue to spend with you.